Wipe duck inside and out with a damp cloth. Combine soy sauce, sherry, sugar and salt; rub mixture over duck inside and out.
Cut scallion stalk in 1/2-inch sections. Crush garlic. Mince ginger root.
Heat oil in a large heavy pan. Stir-fry scallion, garlic and ginger to soften slightly. Add duck and brown quickly on all sides.
Drain canned pineapple. Reserve juice and combine with water, then pour over duck. Top bird with pineapple slices. Bring to a boil, then simmer, covered, until done about 1-1/2 hours.
Transfer duck to a serving platter, leaving liquids in pan. Cut each pineapple slice in four and arrange all decoratively on duck.
Reheat duck liquids. Meanwhile blend cornstarch and remaining cold water to a paste, then stir in to thicken duck liquids. Pour over duck and serve.Servings: 4 to 6