1 large onion, chopped
800 g skinless fillets of fish such as salmon, cod, whiting, hake or haddock
150 ml white wine
1 lemon, juice only
125 g butter, diced
salt and freshly ground black pepper
150 g mushrooms, sliced
225 ml double cream
1 heaped tbsp Dijon mustard
4 tbsp finely chopped mixed herbs
1 kg mashed potatoes
Preheat the oven to 180C/Gas 4/350F.
2. Place the onion in the bottom of a large saucepan and lay the fish on top in an even layer. Pour in the wine add the lemon juice scatter with 100g of the butter and season with salt and pepper. Cover with a lid and simmer on a low heat for 15–20 minutes or until the fish is cooked.
3. In the meantime, melt the remaining butter in a small frying pan and sauté the mushrooms on a gentle heat for 5–6 minutes or until softened. Season well with salt and pepper.
4. Once cooked and using a slotted spoon, carefully transfer the fish from the saucepan (leaving the onions and cooking liquid in the pan) to a 22cm square ovenproof dish.
5. Add the cream to the onions and cooking liquid in the pan and continue to simmer, with the lid off, for 10–15 minutes or until the sauce is reduced and thick enough to coat the back of a spoon. Stir in the mustard, herbs and sautéed mushrooms and check the seasoning.
5. Pour the sauce over the fish in the dish and spoon over the mashed potato, spreading with the back of a spoon or fork. Alternatively, pipe the mash over the fish with a piping bag and nozzle for a more professional-looking finish. The fish pie can be prepared to this stage, left to cool and then placed in the fridge overnight until ready to bake.
6 Bake in the oven for about 30 minutes or until bubbling and golden on top. If cooking from chilled then bake for about 40 minutes instead. Serve immediately.