Heat the oven to 400°F/200°C. Shape and tie the lamb as a crown roast. Slit the meat and stud it with garlic; set it in a roasting pan. Spoon over the oil and sprinkle with salt, pepper, and rosemary. Roast the meat in the oven for 30-35 minutes to 125°F/51°C on a meat thermometer for rare lamb, and for 40-45 minutes, to 140°F/60°C, for medium-cooked lamb. Meanwhile, coo