1. Cut scallion in 1/2 inch sections. Slice ginger root. Combine sugar, sherry, soy sauce and salt.
2. Heat oil. Add ginger and scallion and stir-fry a few times. Stir in sugar sherry mixture to blend. Add pork and cook over low heat, turning constantly for even coloring.
3. Meanwhile bring water to a boil in another pan; then add to pork. Simmer, covered, until done (about 1 1/2 hours). Separate mustard cabbage leaves.
4. Remove pork, but not liquids, from pan. Add cabbage leaves and cook, uncovered, 20 minutes, stirring occasionally. Meanwhile cut pork in slices 1/4 inch thick.
5. Drain cabbage. Transfer to a serving platter. Arrange pork slices on top. Pour gravy over and serve.