Step 1If you’re using the preserved vegetables, put them in a large bowl and add enough cold water to cover. Let stand at room temperature until rehydrated and softened, 2 to 4 hours.
Step 2Meanwhile, fill a large saucepan with water and add the scallions and garlic. Bring to a rolling boil over high heat, then add the pork. Return to a boil and cook until there’s a lot of scum on the surface. Drain in a colander. Discard the scallions and garlic.
Step 3Combine the soy sauce and sugar in a large Dutch oven. Bring to a boil over medium-high heat, stirring to dissolve the sugar, then put the pork in the mixture skin side down in a single layer. Cook until the skin is the color of dark caramel, about 5 minutes.
Step 4Add the wine and bring to a boil. Add enough water to just cover the meat and bring to a boil over high heat. While the water comes to a boil, put the cassia, both cardamoms, mace, and fennel in a tea strainer or wrap in cheesecloth and tie shut. Nestle into the boiling liquid.
Step 5Reduce the heat to low, cover, and simmer until a chopstick inserted into a piece of pork slides easily through meat, fat, and skin, about 2 ½ to 3 hours.
Step 6Lift the greens out of the water and squeeze out excess water with your hands. Trim off and discard the tough ends, then cut the stems and leaves into ½-inch dice. Fold into the bubbling braise and simmer, uncovered for 30 minutes.
Step 7Taste the braise. The vegetables are salty, so you may want to add a pinch of sugar. Stir in a little a time, tasting after each addition. Serve hot.