Carefully wash chilies; remove seeds, and drain. Beat egg whites until stiff, but not dry. Beat egg yolks until thick and creamy. Blend in flour, salt, and pepper. Fold in beaten egg whites. Gently spread half of the egg mixture in greased baking dish. Place all of chilies opened flat over egg mixture. Cover with cheese slices. Top with remaining egg mixture. Bake 25 minutes. Serve hot with Chilies Relleno Sauce.