Set a two-tiered bamboo steamer in a wok or in a large saucepan or skillet over 2 inches of boiling water; alternatively, use a large, wide steamer.
In a small bowl, blend the sake, Riesling and oyster sauce. Arrange the scallops on two 9-inch glass pie plates and season them with salt and pepper. Spoon the sauce mixture over the scallops. Steam over high heat until the scallops are just cooked though, about 3 minutes. (If using a large, wide steamer, steam the scallops in 2 batches.)
Meanwhile, heat the oil in a medium skillet until almost smoking. As soon as the scallops are done, add the scallions and ginger to the hot oil, stir and remove from the heat. Spoon the mixture over the scallops and serve right away.