Preheat oven to 400 F. Wipe fish with damp cloth inside cavities and outside. Put a wide, shallow baking pan in oven to preheat.
Combine softened butter, ham, onions and olives; mix well. Divide evenly among fish, tucking stuffing into each fish cavity. Coat fish with flour; shake off excess. Arrange fish, single layer, on wax paper within reaching distance of oven. Remove pan from oven. Put butter in pan and swirl until melted (margarine should be 1/8 deep). Lay fish in pan, one at a time, turning to coat with melted butter. Return pan to oven and bake uncovered until fish flakes easily with a fork (approximately 10 minutes per inch thickness). When cooked, transfer fish to plates; salt and pepper to taste. Makes 4 servings.