01 - Preheat the oven to 400°F.
02 - Combine minced garlic, rosemary, thyme if using, kosher salt, black pepper, olive oil, and lemon zest in a small bowl to form a paste.
03 - Pat the lamb dry. Make small incisions throughout the meat. Rub the marinade paste thoroughly over the lamb, pushing some into the incisions.
04 - Place sliced onions, carrots, and celery in a roasting pan to create a bed. Set the lamb on top with the fat side facing up.
05 - Roast the lamb at 400°F for 20 minutes to develop a crust.
06 - Lower the oven temperature to 350°F. Roast an additional 1 hour and 10 minutes or until the internal temperature reaches 135°F for medium-rare or 145°F for medium.
07 - Remove the lamb from the oven and tent loosely with foil. Let rest for 15 minutes before carving.
08 - Slice the lamb and serve alongside roasted vegetables. Garnish with fresh rosemary sprigs and lemon wedges if desired.