Rinse the fish under cold water and pat dry. Make four or five 1-1/2-inch incisions to the bone on each side of the fish. Put the scallions and ginger inside the cavity.
If you don't have a steamer, use a roasting pan with a heatproof platter or rack to support the fish above the boiling water. Place the fish on the steamer, cover, and cook 20 - 25 minutes, until the meat is opaque.
In a small saucepan, whisk together the vegetable oil, sesame oil, soy sauce, and sherry. Heat and spoon over each portion of fish.