Rub the garlic over the pork belly. Place on a large wooden board, skin-side up, score it, and dry thoroughly.
Sprinkle the sea salt and pepper all over the scored skin.
Preheat the oven to 230C/Gas mark 8. Transfer the belly of pork to a large roasting tray greased with olive oil, and place on the top shelf of the hot oven. Roast at a high heat for about 30 minutes, or until hard crackling has formed, then turn the heat down to 190C/Gas mark 5 and cook for 2 ½ to 3 hours. Leave to rest.
Deglaze the roasting tray with the sherry, scraping the juices off the bottom of the tray. If it tastes too strong, add a splash of water.
Start preparing the fennel gratin about half an hour before the belly is ready. Keep the oven at the same temperature of 190C/Gas mark 5.
Cut the fennel bulbs into six lengthways so that the individual pieces are held together by
the central core. Blanch then drain well.
Mix the fennel with the cream, half the Parmesan and the garlic. Season with salt and pepper and put in a shallow baking dish. Scatter the remaining Parmesan on top, cover with foil and place in the oven.
Remove the foil after about 20 minutes, then bake for a further 10 minutes.
To serve, slice the pork belly and place on a platter. Serve with the gratin on the side.