800g King Edward potatoes, peeled and cut into chunks
Preheat the oven to 200°C, gas mark 6. Put the salmon in a deep frying pan and pour over 300ml of the milk. Season and bring to a simmer. Cover and cook for 7-10 minutes, until thesalmon is just opaque. Drain the fish, reserving the milk, and set aside to cool.
Heat 50g of the butter in a saucepan and stir in the flour. Mix to a smooth paste and cook for 1-2 minutes. Remove from the heat and gradually stir in the wine and the reserved milk. Return to the heat and bring to the boil, stirring constantly. Reduce the heat and simmer for 2 minutes. Check and adjust the seasoning, then remove from the heat.
Flake the fish, discarding any skin and bones, and gently fold into the sauce, along with the capers and prawns. Spoon the mixture into a 1.5 litre deep, ovenproof dish and set aside.
Boil the potatoes for 10-15 minutes, until tender. Drain well and return to the pan. Mash well until smooth and stir in the remaining butter and milk. Season and spoon on top of the fish mixture. Spread out with a fork and bake for 30-35 minutes, until the potato is golden.