Heat fat, add onions and mushrooms. Cook until tender and remove from pan. Blend flour with fat, add seasonings, let boil 2 minutes and add milk gradually. Cook oysters in their own liquor until edges curl and add oysters and liquor to creamed mixture. Add mushrooms, onion and eggs; stir in sherry. Turn into greased casserole and bake at 200ºC for 15 minutes. Serve on toast or pastry shells. Serves 4 or 5