Slice the pork belly thinly and marinate together with the catfish using oyster sauce, brown sugar, fish sauce, black pepper, and crushed garlic and shallots. Let the fish absorb the flavors for about 1 hour before cooking. Heat oil in a pan, sauté the garlic and shallots, then add the pork belly and cook until slightly browned. Add the catfish and remaining marinade. Bring to a boil and let the flavors infuse into the fish.

Add the coconut water and whole chili peppers, cover, and simmer on low heat until the liquid is almost reduced. Taste and adjust seasoning if needed before transferring to a clay pot. Lightly coat the clay pot with a bit of oil, place the pork belly at the bottom, then add the catfish on top. Continue to simmer on very low heat until the pork is tender and the sauce is just thickened. Turn off the heat when the sauce is just right.