Arrange fish fillets in a single layer in a shallow greased baking dish. Sprinkle with salt and pepper and brush with the butter.
Broil in a preheated broiler about 5 minutes, or until fish begins to flake.
Drain liquids from baking dish carefully to avoid breaking fillets.
While the fish si broiling, start to prepare the following:
Mix the mayonnaise, lemon juice and onion together.
Beat the egg whites until stiff and fold into the mayonnaise mixture very gently. Spread over the broiled fillets. Sprinkle with the slivered almonds. Return to broiler until the top is lightly browned and a little puffy.