Pound the garlic, coriander root, and black peppercorns into a coarse paste using a mortar and pestle.
Place the pork belly in a pot and add enough water to cover. Bring to a boil, reduce to a gentle simmer, and cook for 15–20 minutes until tender. Drain and set aside.
Heat the oil in a wok or large skillet over medium heat. Add the shallots, spice paste, and palm sugar. Cook, stirring frequently, until fragrant and the palm sugar begins to caramelize.
Stir in the dark soy sauce, oyster sauce, fish sauce, and water until combined.
Add the cooked pork belly and simmer for about 20 minutes, stirring occasionally, until the sauce has reduced into a thick, sticky glaze that coats the pork.
Serve immediately with steamed jasmine rice.