01 - Preheat oven to 325°F (160°C).
02 - Combine minced garlic, rosemary, thyme, parsley, oregano, lemon zest and juice, kosher salt, black pepper, and olive oil in a mixing bowl to form a paste.
03 - Pat lamb shoulder dry with paper towels and score the fat in a crisscross pattern using a sharp knife.
04 - Rub the herb paste thoroughly over the entire lamb, pressing it into the scored cuts.
05 - Place sliced onions, carrots, and celery evenly in the base of a large roasting pan.
06 - Set the lamb shoulder atop the vegetables and pour the broth around (not over) the meat.
07 - Cover pan tightly with aluminum foil and roast in the oven for 2.5 hours.
08 - Remove foil, increase the oven temperature to 400°F (200°C), and roast uncovered for 30 minutes, basting occasionally, until lamb is golden and tender.
09 - Transfer lamb to a platter, loosely cover with foil, and let rest for 15 minutes before carving.
10 - Serve lamb with roasted vegetables and pan juices.