1. Wash the fish well inside and out in cold water and remove any large scales. Wipe the fish dry and with a sharp knife cut 5 mm (1/4 in) deep slits at 1 cm (1/2 in) intervals along both sides.
2. Season with salt inside and out and arrange the carp in an ovenproof dish. Stir together the soy sauce, sherry, oil and sugar and sprinkle over the fish together with the ginger.
3. Trim and wash the onions, cut into 5-cm/2-in lengths and arrange them over the fish.
4. Fill a fish kettle up to the level of the perforations with water and bring it to the boil. Place the fish in the ovenproof dish on the perforated plate of the kettle, then cover and steam the fish for 15 - 20 minutes. Add more water if necessary.
Serve with: noodles and a fresh salad.