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 slices and small sprigs of rosemary into the slits.
03 - Rub the lamb with olive oil, salt, pepper, and lemon zest.
04 - Place the carrots, onion, and celery in a large roasting pan. Lay the lamb leg on top, fat side up. Add remaining rosemary and thyme around the lamb.
05 - Squeeze lemon quarters over the lamb and add them to the pan.
06 - Pour the wine or chicken broth into the pan.
07 - Roast uncovered for 20 minutes, then reduce oven temperature to 350°F and continue roasting for 1 hour 20 minutes (about 20 minutes per pound for medium-rare), basting occasionally.
08 - Check doneness with a meat thermometer: 130-140°F for medium-rare.
09 - Transfer the lamb to a platter, cover loosely with foil, and rest for 15-20 minutes before carving.
10 - Strain the pan juices, skim off excess fat, and serve as a sauce alongside the lamb.