The day before, stuff the duck with the onion and celery. Place the duck, breast side up, in a large soup pot with enough water to half cover it. Add the ginger and bring to a boil. Cover and reduce the heat so that it simmers very gently (the bubbles should be lazy not fizzy) for 1 hour.
After 1 hour, turn duck over. Add the sugar, soy sauce, and salt. Continue simmering for another hour. Turn duck once again and simmer until tender and almost falling apart, about another hour. Turn off the heat and when cool enough to handle, remove the duck carefully from the pot (it tends to fall apart after so much cooking) and place in a roasting pan. Cover and refrigerate until the next day.
Pour the broth from the soup pot into a container and chill overnight. A layer of fat will form on the top. Scrape it off and discard it. What remains is delicious in rice and soups and can be frozen for months.
Before serving, bring the duck to room temperature. Preheat your oven to 350 degrees F. Add the sherry and 1 cup of the defatted duck broth to the roasting pan and place in the oven. Roast uncovered for 3- to 45 minutes, basting occasionally with the juices in the bottom of the pan. The duck is done when it is heated through and the skin is crisp and a dark chestnut brown.
Transfer the duck to a serving platter and garnish with lots of watercress, this disguises where legs and wings have fallen off!