Haddock au gratin is the perfect way to ensure your family eats more fish! With succulent haddock fillets nestled on a bed of tender baby spinach, smothered in a creamy leek and bechamel sauce, and topped with sweet potato mash and a layer of golden melted cheese, this dish is as nutritious as it is tasty!
With the new year only a few days away, my resolution for the coming year is to eat more fish, and what better way to do it than with this delicious haddock au gratin!
This oven-baked haddock au gratin is a simple yet satisfying fish pie topped with sweet potato mash and cheese. With just 20 minutes of preparation and 25 minutes in the oven, this is a quick and convenient meal. The short baking time ensures the fish remains flakey and tender, and makes haddock au gratin the perfect choice for a weeknight family meal.
Despite its simplicity, it's not short on taste! The creamy bechamel sauce which has been enriched by the addition of sweet leeks keeps the fish moist and delicious throughout the baking process, while baby spinach not only adds extra nutrients, but also a fresh vibrant flavour to the dish.
The sweet potato mash is the cherry on top (in a manner of speaking). It adds a natural sweetness to the dish and pairs perfectly with the sauce, while the grated cheese adds a little richness.
Whether you're trying to introduce more fish into your diet, or are simply looking for a comforting, homely meal for the family, I'm sure you will love this haddock au gratin!
Firm fish such as cod and haddock are ideal for cooking in the oven as they hold their shape well, and don't fall to pieces. If you'd like to try some of my other oven-baked fish dishes, you might like to try this cheesy cod and broccoli bake or this spicy fish and rice bake in tomato sauce.
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Ingredients
This dish will easily feed 4 people.
- Haddock - skinless haddock fillets. You can use fresh or frozen. If using frozen fillets they should be defrosted first. Substitute with any firm white fish such as hake, cod, Alaskan pollock or kingklip.
- Baby spinach - I prefer to use baby spinach because it doesn't have the bitter taste that you can sometimes get with the larger-leaved variety such as Swiss chard. You could use frozen spinach instead - see FAQ below for details.
- Potatoes and sweet potatoes - these are used for making the sweet potato mash. They should be peeled and cut into chunks.
- Leeks - these are fried in butter and used in the bechamel sauce. They should be thoroughly rinsed to remove any bits of grit and then cut in half lengthways and sliced into semi-circles.
- Butter - for frying the leeks. As this goes into the sauce I would not recommend frying the leeks in cooking oil. You could substitute the butter with margarine if you don't want to use butter.
- Milk - for making the sauce.
- Flour - for thickening the sauce.
- Grated cheese -for sprinkling on top of the sweet potato mash before the dish goes into the oven. I used cheddar - you can substitute it with any cheese that you would normally grate to use in cooking.
- Fish spice - the blend I use is made by Schwartz and is available on Amazon. In the UK you can buy this in any supermarket in the spice aisle. It is a blend of lemon and dill and goes perfectly with fish.
- You will also need salt and black pepper for seasoning. This will be to your own taste.
**You can find the exact ingredient quantities for making haddock au gratin on the printable recipe card at the end of this post**
Instructions
I've listed the instructions in the order I think makes the most sense when making this dish.
Sweet potato mash
Step 1: Place the potatoes and sweet potatoes into a pan of cold lightly salted water. Bring to a boil and boil for about 8 minutes or until the potatoes are soft enough to mash.
Step 2: Drain the potatoes and mash them with a knob of butter and 2 tablespoons of milk. Set aside until needed.
Bechamel and leek sauce
Do this while the potatoes are boiling
Step 3: Melt the butter in a saucepan and add the sliced leeks. Fry them on moderate heat, stirring often, until they have softened. This will take 4 to 5 minutes.
Step 4: Remove from the heat and stir in the flour. Stir in the milk and return the pan to the heat. Continue to stir until the sauce thickens. Season with salt and black pepper to taste and set aside to cool slightly.
Wilt the spinach
Step 5: Rinse the spinach in cold water to remove any dirt or grit and place it in a saucepan. Do not add any extra water - the moisture from rinsing the spinach will be sufficient.
Step 6: Cover the pan with a lid and cook the spinach over moderate heat for 3 to 4 minutes stirring occasionally until it has wilted.
Step 7: Press as much liquid as you can from the spinach (use the back of a spoon to press the spinach in the pan) and arrange the drained spinach on the base of an oven-proof dish.
Assemble the dish
Now it's time to put everything together.
Step 8: Season the fish with fish spice (see ingredient list) and sprinkle lightly with salt and black pepper to taste. Cut the fish into bite-sized pieces and arrange it on top of the spinach.
Alternatively, arrange the whole seasoned fillets on the spinach.
Step 9: Pour the sauce over the fish, taking care to cover the fish completely.
Step 10: Carefully arrange spoonfuls of potato on top of the sauce.
Step 11: Use a fork to spread the potatoes evenly over the sauce.
Step 12: Sprinkle the potato mash with grated cheese.
Bake
Step 13: Place the dish onto a baking tray to catch any drips that might overflow. Bake in a preheated oven at 200C/400F for 25 minutes or until the cheese is melted and golden.
Tips for a successful outcome
Here are my top tips to ensure your haddock au gratin turns out perfectly.
- When making the sweet potato mash, add 2 or 3 tablespoons of milk to the mash so that the potatoes spread easily over the sauce. The mash should not be too dry or you will have difficulty in spreading it. By the same token, don't make the mash too runny or it will be unappealing.
- Depending on your preference either cut the fish into bite-sized pieces or leave the fillets whole before arranging them on the spinach. It won't make any difference to the cooking time.
- The sauce should be of a consistency that allows you to pour it easily over the fish; not too thin or too thick. It should hold its shape but spread easily. If it is too thin, mix another teaspoon of flour with a tablespoon of milk and stir it into the sauce over moderate heat. If too thick, think it down with an extra splash of milk.
Variations
This fish au gratin recipe can be made with any firm fish that cooks to a flakey consistency. It will be particularly good when made with salmon, although I would be inclined to use fresh salmon rather than smoked. If you do make it with smoked salmon I'd love to hear how it turns out!
Instead of leeks, you could add peas into the sauce. In this case I would make the the bechamel sauce (without the leeks) and then stir in a cup of defrosted frozen petit pois before pouring the sauce over the fish.
If you prefer a mustard mash to sweet potato mash, why not simply boil potatoes and add a tablespoon of wholegrain mustard when mashing them. The mild flavour of wholegrain mustard will complement the fish beautifully.
And of course, you could top the pie with a covering of rumbledethumps instead of sweet potato mash to get even more vegetables into the dish!
Serving suggestions
There are lots of vegetables in this haddock au gratin, so you won't need very much in the way of side dishes.
I like to serve this dish with a crisp green salad or a helping of peas. It would also be tasty if served with freshly steamed green beans or broccoli.
Equipment
To make haddock au gratin you will need the following equipment:
- 2 saucepans one for boiling the potatoes for the mash and the other for making the bechamel sauce;
- potato masher to mash the potatoes;
- frying pan with a lid for wilting the spinach;
- sharp knife and chopping board for preparing the vegetables;
- 7" x 9" (18cm x 23cm) ovenproof casserole dish for assembling and baking the haddock au gratin. The size of the dish is approximate - you could use one slightly larger if necessary (but not smaller or your ingredients won't fit).
- Baking sheet (optional) - to place under the casserole dish to catch any drips.
Storage
Leftover haddock au gratin can be kept covered in the refrigerator for up to 2 days. To reheat, cover the dish with a layer of tinfoil and heat it for 10 minutes in a hot oven (200C/400F).
I don't recommend freezing this dish as they sauce may split when frozen and defrosted.
FAQ
Allow the spinach to defrost then squeeze out as much water as possible. There is no need to cook the spinach as it will cook in the oven along with the fish.
Frozen spinach contains a lot of water so use the same weight of frozen spinach as given in the recipe for fresh. Once it has been defrosted and drained you should have sufficient spinach to spread on the bottom of your baking dish.
You can prepare this dish in advance and store it in the refrigerator for up to 2 days. Cook for the time and temperature given in the recipe.
Save for later
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Related recipes
Visit my fish and seafood recipes page for other delicious ideas for cooking fish. In the meantime, here are a few you might enjoy:
📋The recipe
Haddock au gratin
(Click the stars to rate this recipe)
Equipment
- 2 medium saucepans
- Potato masher
- Frying Pan
- Sharp Knife
- Chopping Board
- 7" x 9" ovenproof casserole dish 18cm x ;23 cm
- Baking sheet optional
Ingredients
- 1 pound / 450 grams frozen haddock fillets defrosted
- 1 teaspoon fish spice or more to taste (I use Schwartz)
- ½ teaspoon salt or to taste
- ½ teaspoon ground black pepper or to taste
- 3 ounces / 100 grams baby spinach leaves
Sweet potato mash
- 3 medium potatoes peeled and cut into chunks
- 1 medium sweet potato peeled and cut into chunks
- ½ teaspoon salt or to taste
- ½ tablespoon / 10 grams butter
- 2 tablespoons milk
- ½ cup / 50 grams grated cheddar cheese
Leek bechamel sauce
- 1 large leek washed, sliced in half lengthwise and cut into semi-circles.
- 2 ounces / 60 grams butter
- 1½ cups / 360 ml milk full-fat or semi-skimmed
- 2 tablespoons / 20 grams flour
- ½ teaspoon salt or to taste
- ½ teaspoon ground black pepper or to taste
Instructions
Sweet potato mash
- Place the potatoes and sweet potatoes into a pan of cold lightly salted water. Bring to a boil and boil for about 8 minutes or until the potatoes are soft enough to mash.3 medium potatoes, 1 medium sweet potato, ½ teaspoon salt
- Drain the potatoes and mash them with a knob of butter and 2 tablespoons of milk. Set aside until needed.½ tablespoon / 10 grams butter, 2 tablespoons milk
Bechamel and leek sauce (make while the potatoes are boiling)
- Melt the butter in a saucepan and add the sliced leeks. Fry them on moderate heat, stirring often, until they have softened. This will take 4 to 5 minutes.1 large leek, 2 ounces / 60 grams butter
- Remove from the heat and stir in the flour. Stir in the milk and return the pan to the heat. Continue to stir until the sauce thickens. Season with salt and black pepper to taste and set aside to cool slightly.1½ cups / 360 ml milk, 2 tablespoons / 20 grams flour, ½ teaspoon salt, ½ teaspoon ground black pepper
Spinach
- Rinse the spinach in cold water to remove any dirt or grit and place it in a saucepan. Do not add any extra water - the moisture from rinsing the spinach will be sufficient.3 ounces / 100 grams baby spinach leaves
- Cover the pan with a lid and cook the spinach over moderate heat for 3 to 4 minutes stirring occasionally until it has wilted.
- Press as much liquid as you can from the spinach (use the back of a spoon to press the spinach in the pan) and arrange the drained spinach on the base of an oven-proof dish.
Assemble and bake
- Preheat the oven to 200°C / 400°F
- Season the fish with fish spice (see ingredient list in the post) and sprinkle lightly with salt and black pepper to taste. Cut the fish into bite-sized pieces and arrange it on top of the spinach. You can leave the fillets whole if you prefer.1 pound / 450 grams frozen haddock fillets, 1 teaspoon fish spice, ½ teaspoon salt, ½ teaspoon ground black pepper
- Pour the sauce over the fish, taking care to cover the fish completely.
- Carefully arrange spoonfuls of sweet potato mash on top of the sauce.
- Use a fork to spread the potatoes evenly over the sauce.
- Sprinkle the potato mash with grated cheese.½ cup / 50 grams grated cheddar cheese
- Place the dish onto a baking tray to catch any drips that might overflow. Bake in a preheated oven for 25 minutes or until the cheese is melted and golden.
- Serve with a crisp green salad and/or your choice of vegetables.
Notes
- When making the sweet potato mash, add 2 or 3 tablespoons of milk to the mash so that the potatoes spread easily over the sauce. The mash should not be too dry or you will have difficulty in spreading it. By the same token, don't make the mash too runny or it will be unappealing.
- Depending on your preference either cut the fish into bite-sized pieces or leave the fillets whole before arranging them on the spinach. It won't make any difference to the cooking time.
- The sauce should be of a consistency that allows you to pour it easily over the fish; not too thin or too thick. It should hold its shape but spread easily. If it is too thin, mix another teaspoon of flour with a tablespoon of milk and stir it into the sauce over moderate heat. If too thick, think it down with an extra splash of milk.
- You can also make this dish with frozen spinach. Allow it to defrost, squeeze out as much moisture as possible and arrange it on the base of your casserole. There is no need to cook frozen spinach before using in this recipe.
Nutrition
I am not a nutritionist. The nutrition information has been calculated using an on-line calculator, and is intended for information and guidance purposes only. If the nutrition information is important to you, you should consider calculating it yourself, using your preferred tool.
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Anne Thomas
I just came across this recipe for haddock au gratin and without having made it yet (on the menu this weekend) I know it's going to be delicious! How do I know that? I just made Veronica's prawn and scallop au gratin last weekend, and it was so good! I am confident this recipe will be just as delicious. Love the inclusion of leeks in the bechamel sauce. Yum!
VJ
Thanks for the comment Anne, much appreciated! You're also right about the leeks - they add a whole new dimension to the sauce. Enjoy!