Preheat oven to 160°C (320°F). Arrange the garlic over the base of a baking dish. Rub the olive oil all over the pork belly and rub the salt into the skin. Place skin-side down on top of the garlic and cook for 3 hours. Increase the heat to 180°C (355°F). Turn over the pork, add the sage and cook for a further 30 minutes or until the skin is golden and crunchy.