In a bowl, mix the garlic, rosemary, thyme and oil. Season the lamb with salt and pepper and rub the herb oil all over the meat. Let stand at room temperature for 1 1/2 hours or refrigerate for at least 3 hours or overnight.
Bring the lamb to room temperature if necessary. Light a grill. Grill the lamb over moderate heat, turning once, until browned outside and an instant-read thermometer inserted in the meat registers 125° for medium-rare, about 8 minutes per side. Transfer to a carving board and let rest for 10 minutes. Thinly slice the lamb across the grain and serve.