Trim away the crescent-shaped muscle from each scallop, then put the scallops into a non-aluminium bowl and add the soy sauce and sherry. Leave to marinate for 30 minutes or so, stirring from time to time. Meanwhile, mix together the sauce ingredients.
Heat the oil in a large frying pan or wok and stir-fry the onion for about 1 minute. Tip in the ginger, scallops and marinade and stir-f