Heat oil in a large pot. Saute onion and garlic until soft, about 2 to 3 minutes. Add roast pork belly (or raw pork shoulder or a mix of the two). Cook for another 2 to 3 minutes.
Pour water into the pot. Cover and bring to a boil.
Add pickled mustard greens, Chinese mustard greens, dried chilies, assam gelugor (tamarind slices), and dark soy sauce. Any leftovers (roast meats and/or stir fry vegetables) you may wish to include should be added now. Cover and simmer for 1½ hours. Do stir occasionally.
Season with a little salt if necessary. Add plum tomatoes. Cover and continue to simmer for another 15 minutes. Turn off stove.