01 - Heat the oven to 425°F.
02 - Pat the leg of lamb dry with paper towels and make small, deep slits all over with a sharp knife.
03 - Place the sliced garlic into the slits evenly.
04 - Combine rosemary, thyme, salt, pepper, olive oil, and lemon juice in a small bowl, then rub all over the lamb, massaging into the slits and surface.
05 - Set the lamb in a large roasting pan and arrange the potatoes, carrots, and onion around it if using.
06 - Roast at 425°F for 20 minutes. Reduce heat to 350°F, and continue roasting for 1 hour 10 minutes, or until internal temperature reaches 135°F for medium-rare, adjusting time for desired doneness.
07 - Remove the lamb, tent loosely with foil, and let rest for 15 to 20 minutes before carving.
08 - Slice the lamb and serve alongside the roasted vegetables and pan juices.