Braised Pork with Chestnuts

2 lbs. pork shoulder or fresh ham
1/4 of a scallion, thinly sliced on diagonal
fresh ginger, chopped, or ginger powder
18 pieces canned or fresh chestnuts*
6 tablespoons oil
2 tablespoons sherry
6 tablespoons soy sauce
1 & 1/2 cups water
3 tablespoons rock candy or crystalized sugar