"Slow-roasted pork belly until the skin blisters to a crisp. You may also serve this dish with liver sauce (Mang Tomas), from any Asian store, or atchara, a relish made of unripe papaya and vegetables cooked in vinegar, salt, sugar, and spices."
Pat pork belly dry with paper towels. Score pork belly skin in a crosshatch pattern with a sharp knife.
Place pork belly skin side-up in a large roasting pan. Pour fish sauce evenly over pork belly. Season with rock salt and pepper. Let stand at room temperature for 2 hours.
Preheat oven to 400 degrees F (200 degrees C). Cover roasting pan with aluminum foil.
Roast pork belly in the preheated oven for 30 minutes. Reduce oven temperature to 300 degrees F (150 degrees C). Continue baking, basting pork belly with juices every 30 minutes, until very tender, about 4 hours.
Remove aluminum foil and turn on the oven's broiler. Broil pork belly until skin is crisp, about 30 minutes. Remove pork belly from oven. Let rest for 30 minutes. Slice off the bone.
Mix shallots, soy sauce, calamansi juice, sinamak, and chile peppers in a bowl to make dipping sauce. Serve slices of pork belly with dipping sauce.