Lay the rack of lamb flat (skin scored) in a roasting tin with the unpeeled garlic and the rosemary, season with salt and pepper, sprinkle with finely chopped mint. Put in the oven under the grill for 10 minutes. Stand the rack of lamb up in the tray, then cook for another 10 minutes at 180°C (gas mark 6) for rare/pink meat, or 15 minutes for medium rare. Turn off the oven and let the meat sit for an extra 5 minutes. Serve immediately with the cloves of garlic and the meat juices. Season each slice on both sides with salt and pepper.