To prepare the sauce, combine all the ingredients in a small bowl. Set aside.In a mortar and pestle, pound the garlic, coriander roots and coriander seeds to a fine paste. Transfer to a large bowl. Add the pork belly, sugar, soy sauce, oyster sauce and pepper. Mix well. Place in the fridge for at least 1 hour to marinate (or overnight, if possible).
Bring a barbecue or chargrill to medium-high heat. Grill the pork belly for 8-10 minutes each side.
Chop the belly into 2cm-thick pieces. Serve with the tamarind dipping sauce.