1. Place pork and water in a pan, bring to a boil, then cook, covered, 15 minutes over medium heat. Drain, reserving stock.
2. Combine sugar, sherry and soy sauce, and add to pork in pan. Cook over very low heat, turning meat constantly until evenly colored. (If the pan gets dry, moisten with a few drops of reserved stock.)
3. Add remaining stock; bring to a boil, then simmer, covered, 20 minutes. Remove pork and let cool. Cut against the grain in 1/2 inch slices, then in strips about 1 by 2 inches.
4. Place pork slices skin side down in a deep, heatproof bowl. Steam on a rack 1 hour
5. Wash spinach and remove tough stems. Heat oil, add spinach and stir-fry until softened, but still fresh and green (about 2 minutes).
6. Heap spinach over pork in the steaming bowl. Invert bowl into a deep serving dish, so that spinach is at the bottom and pork, skin side up, is on top.