01 - Preheat oven to 400°F.
02 - Pat lamb dry with paper towels. Make small incisions all over and insert garlic halves and rosemary leaves into each cut.
03 - Combine olive oil, thyme, salt, and pepper. Rub mixture evenly over entire surface of lamb.
04 - Scatter onions, carrots, and celery across bottom of roasting pan. Position lamb leg on top of vegetables.
05 - Pour wine or stock into pan, taking care not to pour directly over lamb.
06 - Roast 20 minutes at 400°F to brown, then reduce to 350°F and continue 1 hour 10 minutes until internal temperature reaches 140–150°F for medium-rare. Baste occasionally with pan juices.
07 - Remove from oven, tent loosely with foil, and rest 15–20 minutes before carving.
08 - Strain pan juices and serve alongside sliced lamb.