Haddock, Leek & Mustard Gratin | Easy Creamy Haddock Bake with Leeks & Golden Topping

Haddock, leek and mustard gratin
Haddock, leek and mustard gratin
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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.

Serving Suggestions for Haddock, Leek & Mustard Gratin

  • Roasted Cherry Tomatoes Juicy, blistered tomatoes that add acidity and colour
  • Steamed Green Beans Clean, crisp greens that balance the creamy mustard sauce
  • Tenderstem Broccoli Light, fresh veg that brighten the rich gratin
  • Buttered Peas or Minted Peas Sweet, vibrant peas that pair beautifully with fish
  • Roasted Carrots Sweet, caramelised carrots that complement the savoury leeks
  • Honey‑Glazed Parsnips A cosy, slightly sweet side that works well with mustard
  • Simple Green Salad Crisp leaves with a lemon‑olive oil dressing to cut through the creaminess
  • Sautéed Spinach with Garlic Soft, savoury greens that echo the warmth of the gratin
  • Garlic‑Roasted Kale A deeper, earthy option that complements the mustard
  • Crusty Bread or Warm Baguette Perfect for mopping up the creamy sauce
  • Buttered New Potatoes Light, classic potatoes that keep the plate balanced
  • Crispy Roast Potatoes A golden, crunchy option that adds texture against the soft gratin

Variations: Haddock, Leek & Mustard Gratin

  • Haddock, Leek & Cheddar Gratin — add cheddar to the sauce or topping for a richer, more indulgent finish.
  • Haddock, Leek & Spinach Gratin — fold spinach into the sauce for colour and a soft, silky texture.
  • Haddock, Leek & Sweetcorn Gratin — add sweetcorn for pops of sweetness that brighten the mustardy base.
  • Haddock, Leek & Pea Gratin — stir in peas for gentle sweetness and extra colour.
  • Haddock, Leek & Broccoli Gratin — include small broccoli florets for freshness and extra veg.
  • Haddock, Leek & Bacon Gratin — add crispy bacon for smoky savoury depth that complements the mustard.
  • Haddock, Leek & Herb Gratin — finish with parsley or dill for freshness that lifts the creamy sauce.
  • Haddock, Leek & Garlic‑Rich Gratin — increase garlic for deeper aromatic warmth.
  • Haddock, Leek & Wholegrain Mustard Gratin — swap Dijon for wholegrain mustard for texture and a milder tang.
  • Haddock, Leek & Potato‑Topped Gratin — add thin potato slices on top for a heartier, dauphinoise‑style finish.
  • Haddock, Leek & Tomato Gratin — add chopped tomatoes for a lighter, brighter sauce.
  • Haddock, Leek & Courgette Gratin — include sliced courgette for a light, summery twist.
  • Haddock, Leek & Lemon Gratin — finish with lemon zest for a citrusy lift that cuts through the cream.
  • Haddock, Leek & Parmesan Crumb Gratin — mix Parmesan into the breadcrumbs for a golden, savoury crust.
  • Haddock, Leek & Mixed Seafood Gratin — add prawns or salmon for a more luxurious seafood mix
Haddock, leek and mustard gratin

Haddock, leek and mustard gratin

Haddock, leek and mustard gratin with flaky fish and tender leeks in a creamy sauce, topped with a golden crust, creating a warm, comforting, rustic-looking dish.
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Cook Time 50 minutes
Course Dinner
Cuisine British
Servings 4 people

Equipment

  • Baking tray
  • Saucepan
  • Oven proof dish

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.