Lay the lamb fat side down on a large rimmed baking sheet. Scatter the garlic cloves, rosemary and thyme sprigs and lemon slices around the lamb and roast on the top rack of the oven for about 25 minutes, or until nicely browned. Turn the lamb fat side up and roast for about 25 minutes longer, or until an instant-read thermometer inserted in the thickest part of the meat registers 130° for medium-rare. Transfer the lamb to a carving board and let rest for 10 minutes. Slice the lamb 1/4 inch thick and serve.