01 - Set the oven temperature to 325°F.
02 - Pat the leg of lamb dry and score the fat lightly with a sharp knife.
03 - Combine olive oil, minced garlic, rosemary, thyme, lemon zest, lemon juice, kosher salt, and pepper in a small bowl, then rub evenly over the lamb.
04 - Place onions, carrots, and celery in the base of a large roasting pan and set the lamb on top.
05 - Pour white wine or chicken broth around the lamb in the pan.
06 - Cover the pan tightly with foil or a lid and roast in the oven for 4 hours.
07 - Remove covering, increase oven temperature to 400°F, and roast uncovered for 20 to 30 minutes until a golden crust forms.
08 - Transfer the lamb to a cutting board, loosely cover with foil, and let rest for 20 minutes before slicing.
09 - Strain the pan juices, skim off excess fat, and serve alongside the sliced lamb.