Steam or cook the fish in water until it flakes. Prepare a white sauce with butter, flour, milk and 1/2 teaspoon salt. (Part of the white sauce liquid may be water from the cooked fish) To the white sauce add peas, grated onion, green pepper, and 1/2 teaspoon salt. Place the cooked fish in a buttered casserole; pour the sauce over it. Top with well-seasoned mashed potato. (To freshly cooked and mashed potato, add 1 tablespoon butter, 1/2 cup milk, and salt and pepper to taste.) Bake in a hot oven 400° for 12 minutes or until hot and bubbly.Serves 1.