This haddock, leek and mustard gratin is absolutely delicious — even my family loves it, and they’re not usually big fish eaters. Everything cooks in one dish, the flavours meld beautifully, and the result is a comforting gratin that’s ideal for a relaxed mid‑week dinner. Tender flakes of haddock, sweet softened leeks and a creamy mustard sauce bake together under a golden, bubbling topping that’s simple to make and incredibly satisfying.

Haddock and leeks are a match made in heaven, and the gentle warmth of the mustard brings the whole dish together in the easiest, most delicious way. It’s rich without being heavy, creamy without feeling too indulgent, and a real crowd‑pleasing meal.

Haddock, leek and mustard gratin

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Cook Time 50 minutes mins

Course Main Course

Cuisine British

Servings 4 people

Ingredients

  

• 4 frozen haddock fillets
• 25 g butter
• 4 large leeks, whites only, thinly sliced
• 1 tsp dried thyme
• 150 ml vegetable stock
• salt and pepper

For the crumb

• 80 g panko breadcrumbs
• 20 g grated cheddar cheese
• 1 tsp dried thyme

For the sauce

• 60 g salted butter
• 60 g plain flour
• 600 ml semi-skimmed milk
• 120 g cheddar cheese, grated
• 1 tbsp wholegrain mustard
• 1 tbsp English mustard
• 2 tsp worcester sauce

Instructions

 

• Preheat the oven to 180ºc / Gas 4.
• Place the haddock fillets on the tray and bake in the oven for 15mins until partially cooked.  Set aside to cool slightly.
• Whilst the haddock is cooking, make the crumb.  Mix all the crumb ingredients together and set aside.
• Now make the sauce, melt the butter in a saucepan and when it starts bubbling stir in the flour.  Top tip: by using a large balloon hand whisk, you will find that you will not have any lumps.
• Cook for 30 seconds then gradually start adding the milk, stirring constantly so that the sauce doesn't catch on the bottom of the pan. Bring to a simmer and cook for 2mins, then stir through the cheese until melted. Add both the mustards, worcester sauce, season and set aside.
• Once the fish is cool enough to handle, break up into a shallow dish.
• Increase the oven to 220ºc/gas 7.
• Now cook the leeks, melt the remaining 25g butter in a frying pan, add the leeks and cook for 5mins until they start to soften.  Add the thyme and stock and cook for around 5mins.  Once cooked, gently fold into the haddock. 
• Spoon over the cheese and mustard sauce, scatter over the crumbs and bake in the oven for 15mins until the crumb is golden and the sauce is bubbling.  Serve with peas or green veg.