1. Preheat the oven to 170˚C/Gas 3.
2. Untie the belly of pork if it is rolled and lay it flat on a chopping board. Score the skin evenly in a criss-cross pattern with a sharp knife. Turn the belly skin side down and cut a slit through the thick end of the pork to open it out like a butterfly so that the meat is evenly thick throughout. Rub all over with salt and pepper and a drizzle of olive oil.
A clean and sharp Stanley knife (or craft knife) is the most effective tool for scoring the tough pork skin.
3. Place the garlic, halved side up, on a lightly oiled roasting tray and scatter over the thyme sprigs. Lay the pork belly on top, fat side up. Trickle with a little more olive oil and sprinkle with a little more sea salt. Add a splash of white wine around the pork, cover the meat with a piece of foil and bake for 1½ hours. Remove the foil, baste the pork with the juices and return to the oven, uncovered, for another ½-1 hour until the meat is tender. Continue to baste the pork occasionally with the pan juices.
4. Transfer the pork to a clean chopping board and leave to cool slightly. While still warm, place another tray on top of the pork and weigh down with a few heavy tins to flatten it. Cool completely, then chill for four hours or overnight in the refrigerator to set its shape.
5. Pour off any excess oil from the roasting tray and place over high heat. Deglaze the tray with a generous splash of white wine, scraping the bottom and crushing the heads of garlic with a wooden spoon to release the sediment. Boil the liquid until reduced by half, then add the chicken stock and bring back to the boil until reduced and thickened. Strain the stock through a fine sieve, pressing down on the garlic pulp with the back of a ladle. Adjust the seasoning with salt and pepper.
6. Heat the oven to the highest setting, about 250˚C/Gas 9.
7. Cut the pressed pork into individual portions or squares and pat the skin dry with kitchen paper. Place the pork squares, fat side up, in a roasting tin and drizzle with olive oil and a generous pinch of sea salt. Roast for 15-20 minutes until the skin is golden brown and crisp. Rest the pork for 5 minutes, then serve with the light gravy and accompaniments.