Clean the scallops and cut in half, then simmer in the milk for 15 minutes.
Strain, reserving the liquid.
Heat 1 tablespoon of the butter and stir in the flour, cook for about a minute,
stirring, then add the milk gradually, stirring all the time to avoid lumps. Season
with salt and pepper, add the sliced mushrooms and simmer for about 10 minutes longer.
Add the sherry or wine and finally the scallops. When hot, transfer to an ovenproof
dish and cover with mashed potatoes, making sure they cover the fish right to the
Dot with the remaining butter and bake in a moderate oven, 350 degrees F, for
20 to 30 minutes, or until the top is turning brown.