Put the ginger, lemon grass, lime, sherry, fish sauce and enough water to just cover the fish in a fish kettle or large frying pan. Bring to the boil, then reduce the heat and simmer gently for 5 minutes.
Place the sea bass in the fish kettle or on a large piece of nonstick baking paper if using a frying pan. Lower into the stock, adding more water if necessary so that it covers the fish. Bring the stock to the boil and then turn off the heat. Cover and leave to poach for 15 minutes or until the fish flakes easily when pressed in the centre with a knife.
Meanwhile, make the salsa. Place the tomatoes, lemon grass, ginger, coriander and spring onions in a bowl. Stir through the oil, lime juice and soy sauce and leave to infuse.
Lift the poached sea bass carefully from the cooking liquid on to a plate. Peel away the skin and gently lift the fillets from the bones. Place on a serving dish with the salsa and serve with steamed rice and lime wedges, if liked.