Put the fish in a baking dish and season with salt and pepper.
Pour over 1 cup of the milk and dot with a few flecks of butter, bake in the oven for 15 minutes.
Strain but reserve the cooking juices, remove any skin from the fish and flake the flesh in large pieces.
Make the sauce, melt the rest of the butter in a saucepan, stir in the flour and cook it for 1 minute, start adding the cooking juices bit by bit, stirring all the time, after that add the rest of the milk, if still too thick just add a little more milk.
Season the sauce with salt and pepper.
Mix in to the sauce: fish, prawns, eggs, capers, parsley, peas and celery, add the lemon juice and check the seasoning.
Pour the fish mixture in a 1½ liter greased ovenproof dish.
Mash the potatoes with the butter, sour cream and nutmeg, check the seasoning, and then spread this on top of the fish.