Remove the skin from the fish and cut the flesh into thin shreds.
Place in a bowl and sprinkle with a pinch of salt.
Add the sherry, the egg white and then the cornflour.
Leave the fish to stand, turning it over once or twice.
Cut the celery heart into thin shreds.
Heat 2 tablespoons of oil in a wok and stir-fry the celery with the remaining salt for about 1 1/2 minutes.
Transfer to a warmed serving dish.
Heat the remaining oil in the wok or a deep pan over a moderate heat.
Deep-fry the shredded fish for about 2 minutes, separating them with a pair of chopsticks.
When all the shreds are floating on the surface of the oil, scoop them out with a slotted spoon, drain on absorbent kitchen paper and arrange in the middle of the celery.
Strew the cooked ham on top and garnish with celery leaves.