Trim off excess fat and tough outer skin from the lamb, leaving a thin layer of fat. Make about 10 incisions near the leg bone and insert garlic slivers. Rub the meat with the salt and pepper, then coat with the fat and oil. Massage into the meat. Refrigerate, loosely covered with plastic wrap, for at least 3 hours or overnight. Remove from the refrigerator 2 to 3 hours bef