01 - Preheat your oven to 400°F.
02 - Pat the lamb leg dry with paper towels. Using a sharp knife, make small incisions all over the lamb. Insert garlic halves and rosemary leaves into the incisions.
03 - In a small bowl, mix olive oil, thyme, salt, and pepper. Rub this mixture evenly over the lamb.
04 - Scatter the onions, carrots, and celery in the base of a large roasting pan to form a vegetable bed. Place the lamb leg on top of the vegetables.
05 - Pour the wine or stock into the pan, avoiding pouring directly over the lamb.
06 - Roast in the preheated oven for 20 minutes to brown, then reduce the temperature to 350°F and continue roasting for 1 hour 10 minutes, or until the internal temperature reaches 140–150°F for medium-rare. Baste the lamb occasionally with pan juices.
07 - Remove the lamb from the oven, tent loosely with foil, and rest for 15–20 minutes before carving.
08 - Strain the pan juices and serve as a jus alongside the sliced lamb.