Rinse fish; cut into pieces. In a food grinder or a food processor bowl, grind or process the fish and the onion.
In medium mixing bowl, combine fish, egg, cornstarch, salt, nutmeg, milk, and melted butter. If you like, cover and store in the refrigerator several hours or overnight, or use immediately.
In a large skillet, heat the cooking oil over medium heat. For each fish cake, use a large spoon to drop about 1/3 to 1/2 cup mixture into skillet, forming it into patties and flattening slightly.
Cook fish cakes 5 to 6 minutes or until golden, turning once. Reduce temperature as necessary to prevent over-browning. Remove; drain on paper towels. If you like, garnish with citrus slices and herb sprigs. Makes 4 servings.