Fish gratin may sound like a complicated dish, but it's actually surprisingly easy to make at home. It's also something of an upgrade from your typical dishes of fish pie and fish with vegetables, with a much more interesting flavour and texture.

In one episode of Rick Stein's Long Weekends, chef Rick Stein shared a recipe for cod gratin while visiting Iceland. Rick's preference for cod is perhaps unsurprising given that Cornwall's Padstow, where Rick has focused his culinary empire, is a haven for catching cod. However, the celebrity chef adds that if you don't have any cod, or just don't like it, then this gratin can be made with any white fish.

He says: "You can use any white fish in this gratin recipe, and you can flex your chef muscles over the béarnaise sauce."

This simple fish gratin may take an hour to cook, but it's actually a relatively simple recipe, with just eight ingredients, making it a great choice for both weekends and weeknights.

Ingredients

For the gratin:

• Two leeks, sliced
• Two carrots, sliced
• One onion, chopped
• 40g/1½oz butter, plus extra for greasing
• 600g/1lb 5oz cod loin, skinned and cut into 3cm/1in chunks
• Two tbsp plain flour
• 50ml/2fl oz dry white wine
• Salt and freshly ground black pepper

For the béarnaise sauce

• 70ml/2½fl oz white wine vinegar
• Two shallots, finely chopped
• Two sprigs of fresh tarragon, plus one tsp freshly chopped
• One bay leaf
• Six peppercorns
• Four free-range egg yolks
• 300g/10½oz unsalted butter

Method

Add the cod and flour to the pan and stir over the heat for a few minutes before adding the wine. Let the pan's contents cook until they thicken. Season with salt and pepper to your liking.

Then start preparing the béarnaise sauce. In a saucepan, gently warm the vinegar before adding the shallots, tarragon sprigs, bay leaf, and peppercorns. Let it simmer over a medium heat until the liquid has reduced by at least half, then strain and set aside to cool.

Turn the oven on to 180C/160C Fan/Gas 4 and butter a shallow ovenproof dish ready for the gratin. While the oven heats up break the egg yolks into a bowl and add a teaspoon of water to loosen them. Gradually pour this into the strained, cooled vinegar, stirring continuously.

Transfer this mixture to a heatproof bowl placed over a pan of simmering water. Whisk the mixture until it expands in volume and is thick enough to coat a spoon.

Melt the unsalted butter in a new saucepan. Then, remove the bowl of egg yolk and vinegar mixture from the heat and slowly add the melted butter in a steady stream, whisking constantly.

Continue whisking until the mixture becomes thick and smooth. Stir in the chopped tarragon and season with salt and pepper as per your taste.

Transfer the cod and vegetable mix into the ovenproof dish, pour the béarnaise sauce over it, and bake for 15 to 20 minutes.