01 - Set the oven to 325°F (160°C) and allow it to reach temperature.
02 - Pat the leg of lamb dry and lightly score the fat surface with a sharp knife.
03 - In a small bowl, combine olive oil, minced garlic, rosemary, thyme, lemon zest, lemon juice, kosher salt, and black pepper. Rub the mixture evenly all over the lamb.
04 - Place quartered onions, carrot chunks, and celery chunks in the base of a large roasting pan.
05 - Set the seasoned lamb atop the vegetables. Pour the white wine or chicken broth around the lamb in the pan.
06 - Cover the roasting pan tightly with its lid or heavy-duty foil. Roast in the oven for 4 hours to develop deep flavor and tender texture.
07 - Remove the cover and increase oven temperature to 400°F (200°C). Continue roasting uncovered for 20 to 30 minutes until a golden crust forms.
08 - Transfer the roast to a cutting board and cover loosely with foil. Let it rest for 20 minutes to redistribute juices.
09 - Strain the roasting pan juices, skim off excess fat, and serve alongside sliced lamb.