Have four salmon steaks cut just below the head of the fish. Put some bits of salt pork on a fish-sheet (the cover of a tin cracker box with edge pounded flat answers); on the pork lay two of the steaks, the open ends toward each other. Remove the stems from half a pound of fresh mushrooms and peel the caps. Melt three or four tablespoonfuls of butter in a frying-pan; add a tablespoonful of grated onion and the caps, let cook three minutes. Set the caps on the salmon; add salt and pepper with bits of butter; above the mushrooms set the other slices of salmon, with open ends toward each other; pour the butter (if any) from the mushroom pan over the fish. Bake about half an hour, basting four or five times with melted butter. Meanwhile, chop the mushroom stems and peelings fine; melt two tablespoonfuls of butter, and in it cook and stir the chopped material until the butter is absorbed, add a cup and a half of cream and let simmer very gently until the fish is cooked. Season with salt and pepper and use as a sauce for the fish.