01 - Preheat the oven to 400°F.
02 - Pat the lamb leg dry with paper towels. Use a sharp knife to make small incisions all over the lamb.
03 - In a small bowl, combine olive oil, minced garlic, chopped rosemary, salt, pepper, and lemon zest.
04 - Rub the marinade thoroughly over the lamb, pushing some into the incisions for maximum flavor.
05 - Arrange onions, carrots, and celery in a large roasting pan to create a bed. Place the lamb leg on top.
06 - Roast for 20 minutes to develop a golden crust.
07 - Reduce oven temperature to 340°F. Continue roasting for about 1 hour 20 minutes (15 minutes per 1 lb for medium-rare), basting once or twice with the pan juices.
08 - Check internal temperature: 130–140°F for medium-rare, 150°F for medium.
09 - Remove from oven, tent loosely with foil, and let rest for 20 minutes before carving.