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 meat.
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, pressing some into the incisions for maximum flavor penetration.
05 - Arrange onions, carrots, and celery in a large roasting pan to create a vegetable bed. Place the seasoned lamb leg on top.
06 - Roast for 20 minutes to develop a golden, flavorful crust.
07 - Reduce oven temperature to 340°F and continue roasting for approximately 1 hour 20 minutes (15 minutes per pound for medium-rare), basting once or twice with the pan juices.
08 - Verify internal temperature: 130-140°F for medium-rare, 150°F for medium.
09 - Remove from oven, tent loosely with aluminum foil, and let rest for 20 minutes before carving to allow juices to redistribute.