01 - Preheat your oven to 400°F for initial high-heat roasting.
02 - Pat the lamb leg dry with paper towels. Using a sharp knife, make small incisions all over the lamb. Insert garlic slices and small sprigs of rosemary into the slits.
03 - Rub the lamb thoroughly with olive oil, kosher salt, freshly ground black pepper, and lemon zest, ensuring even coverage.
04 - Place the carrots, onion, and celery in a large roasting pan to create a bed. Lay the lamb leg on top, fat side up. Distribute remaining rosemary and thyme around the lamb.
05 - Squeeze the lemon quarters over the lamb and place them in the pan. Pour the white wine or chicken broth into the bottom of the roasting pan.
06 - Roast uncovered at 400°F for 20 minutes to develop a browned exterior.
07 - Reduce oven temperature to 350°F and continue roasting for 1 hour 20 minutes (approximately 20 minutes per pound for medium-rare), basting occasionally with pan juices.
08 - Verify internal temperature using a meat thermometer: 130–140°F for medium-rare.
09 - Transfer the lamb to a serving platter, cover loosely with aluminum foil, and allow to rest for 15–20 minutes before carving to retain juices.
10 - Strain the pan juices through a fine mesh sieve, skim off excess fat from the surface, and serve warm alongside the carved lamb.