In a saucepan, melt butter and stir in flour until smooth. Remove from heat. Gradually stir in ½ of the milk. Put back on heat and beat until smooth and shiny. Gradually add remaining milk, salt, pepper and onion. Cook, stirring until smooth and thick. Pour over fish. Sprinkle with bread crumbs and cheese. Bake at 375 F for 15 to 30 minutes.