250 g (8 oz) white fish fillet, such as coley, pollack or haddock, cut into bite- sized pieces
250 g (8 oz) salmon fillet, cut into bite-sized pieces
400 ml (14 fl oz) hot milk
50 g (2 oz) butter
50 g (2 oz) flour
100 g (3 1/2 oz) Cheddar cheese, grated
2 teaspoons lemon juice
2 tablespoons chopped chives
100 g (3 1/2 oz) small cooked peeled prawns (optional)
salt and pepper
Preheat the oven to 180°C (350°F), Gas Mark 4. Cook the potatoes in a large saucepan of lightly salted water for 6–7 minutes. Drain and set aside to cool slightly.
Meanwhile, place the fish in a deep-sided frying pan. Pour over the hot milk and bring to the boil. Reduce the heat and simmer gently for 3–4 minutes, until the fish is just cooked. Strain the milk into a jug and transfer the fish to an ovenproof dish.
Place the butter and flour in a saucepan and warm gently to melt the butter. Stir over the heat to cook the flour for 2 minutes, then add the milk a little at a time, stirring well to incorporate. Stir over the heat for 2–3 minutes, until thickened, then remove from the heat and stir in half the cheese, the lemon juice and chives, and season to taste. Pour the sauce over the fish, add the prawns, if using, and stir gently to coat.
Wearing rubber gloves to protect your hands from the heat, grate the potatoes coarsely and scatter over the fish. Sprinkle with the remaining cheese and cook in the preheated oven for 15–20 minutes, until the topping is golden and crispy.