Heat butter in skillet, saute onions, separate into rings, for 4 - 5 minutes until limp, set aside. Combine cheese and bread crumbs, set aside. Arrange fish portions on oiled rack of broiler pan. Season with salt and pepper. Spread half the mayo over fish. Place fish under preheated broiler, about 4 inches from source of heat. Broil for 10 minutes per inch thickness of thickest portion, turning once 3 minutes before end of cooking time to spread with remaining mayo and top with onions and cheese/crumb mixture. Serves 4. Especially nice with firm white fish fillets, such as Snapper or Cod.