Slice the garlic into slivers. With a sharp knife make incisions in the pork belly, on all sides except the skin (unless it’s scored, which I don’t see many benefits of; unscored skin peels off easier for carving) and push the garlic slivers in, as many as you can.
Season the pork belly all over generously with the marjoram and pepper and drizzle with the black bean paste, soy sauce and honey. Rub the meat with the sauces on all sides and leave to marinate for at least an hour - overnight would best.
Preheat the oven to 170C/325F/gas 3. Slice the onion, peeled or unpeeled, into 4 rings, place them on the bottom of a roasting tray and sit the pork belly on the onion, skin side up. Pour the sherry into the tray, tent it with foil so that it’s sealed but doesn’t touch the meat.
Roast for 2 ½ hours, turn the heat up to 180C/350F/gas 4, take the foil off and roast for another hour, until the skin is crisp and charred around the edges. Let the meat rest for 15 minutes before carving.
