Cut the pork belly into large, uniform pieces. Bring a pot of water to a boil, add the pork, cover, and cook for 15 minutes. Drain well.
Pound the coriander root, black peppercorns, and garlic together in a mortar and pestle until you have a rough paste.
Heat the oil in a deep wok or skillet over low heat. Carefully add the pork belly and sear for 2–3 minutes.
Add the pounded aromatics and stir to coat the pork. Add the pickled mustard greens, golden mountain seasoning sauce, dark soy sauce, and palm sugar.
Pour in enough water to just cover the pork. Cover and simmer over low heat for about 1 hour, turning the pork every 15 minutes, until tender and the sauce has thickened. If the pork is still chewy, continue simmering until tender.
Serve hot with steamed jasmine rice and spoon some of the sauce over the top.