01 - Set the oven to 425°F to prepare for roasting.
02 - Combine rosemary, garlic, thyme (if used), lemon zest, kosher salt, black pepper, and olive oil in a small bowl to form a paste.
03 - Pat the leg of lamb dry with paper towels. Using a sharp knife, make small deep incisions throughout the meat and insert garlic slices into each.
04 - Rub the herb paste evenly over the entire surface of the lamb, ensuring thorough coverage.
05 - Spread onions, carrots, and celery evenly in a large roasting pan, then place the lamb on top with the fat side facing upward.
06 - Pour white wine or chicken broth into the pan, avoiding contact with the lamb to preserve the herb coating.
07 - Roast for 20 minutes at 425°F to develop a crust.
08 - Reduce oven temperature to 350°F and continue roasting for 60 to 75 minutes, or until an internal temperature of 135–140°F is reached for medium-rare, measured at the thickest part of the meat.
09 - Remove lamb from oven, loosely cover with aluminum foil, and rest for 15 to 20 minutes to allow juices to redistribute.
10 - Strain the pan juices, skim off excess fat, and serve alongside the carved lamb. Optionally, reduce the strained juices with a splash of wine to create a sauce.