Pat the pork belly dry and score the skin with a sharp knife in both directions, without cutting all the way through to the meat. Rub the garlic clove over the surface of the pork, drizzle with oil and rub all over. Pull the leaves off a couple sprigs of thyme and sprinkle overtop; drizzle with a little more oil. Let the meat stand for about 30 minutes.
Meanwhile, preheat the oven to 450?F and place the onions in the bottom of a roasting pan. Place the pork belly on top and place a couple sprigs of thyme and the halved garlic around it; drizzle the exposed garlic with a little oil and roast for 30 minutes. Turn the heat down to 300?F and continue to roast for 2 1/2 hours, until the meat is tender and the skin is crisp. If you like, toss a few new potatoes into the pan around the meat, shaking it up to coat them with the rendered fat, about three quarters of the way through the cooking time.
Let rest on a cutting board before slicing. Serves 4-6.