Fill a large pot with about two inches of water. Place a steamer on top. If no steamer is available, place a small heat proof baking dish or ramekin that can keep a large plate high enough not to touch the water.
Open the lid, and spoon the sauce all over the fish. Cover, and steam. It’s done if the flesh separates from the cartilage easily. Remove from the pot and place the optional scallion while the fish is still hot.