Crush the garlic cloves, mix a paste with pepper, salt and lemon zest. Brush the loin with it and let it soak for about an hour. Tie the rosemary sprigs along the loin so that the loin is completely protected. Fry with a little oil in a pan, first spicy, then over low heat for a maximum of half an hour. Take the loin out of the pan and let it rest on a board for five minutes to allow the meat juice to spread out. Remove the rosemary sprigs and cut the loin