Heat oil in a large pot. Add crackly roast pork belly, onion, and garlic. Fry until onion and garlic are lightly brown, about 3 minutes.
Pour about 10 cups (2.4 liters) of water into the pot. Add pickled mustard greens, dried chilies, and assam gelugor (tamarind slices).
When it comes to a boil, add salt, dark soy sauce and as much mustard greens as the pot can accommodate. Put the lid on and allow vegetables to cook down.
Add remaining mustard leaves and tomatoes.
Reduce heat and allow it to simmer for about 1½ to 2 hours.
1. If you do not have roast pork belly, use 6 slices of bacon and 1 pound of pork sirloin cut into bite size pieces. These should be added at the beginning together with onions and garlic.2. Asam gelugor (tamarind slices) may be substituted with 2 teaspoon (or more depending on taste) of asam jawa (tamarind paste). If you can’t find tamarind slices or tamarind paste, add in lime juice. I have tried this and it tastes just as good.3. Any leftovers that you may wish to include should be added to the pot when vegetables are being added.