For the tamarind sauce: Mix together the tamarind water, coriander, sugar, chili flakes, galangal and garlic and set aside.
For the pork belly: Pound the garlic, coriander seeds and coriander roots together into a fine paste in a mortar and pestle.
Place the pork belly and the paste into a large mixing bowl. Add the oyster sauce, soy sauce, pepper and sugar. Mix well and marinate in the refrigerator for at least 1 hour or overnight for a better result.
Bring a grill to medium-high heat and grill each side for 8 to 10 minutes.
Chop the belly into 3/4-inch-thick pieces and serve with tamarind dipping sauce.