01 - Preheat oven to 325°F.
02 - Combine garlic, rosemary, thyme, parsley, oregano, lemon zest, lemon juice, salt, pepper, and olive oil in a small bowl until paste forms.
03 - Pat lamb shoulder dry, then score the fat in a criss-cross pattern with a sharp knife.
04 - Rub herb paste thoroughly over entire lamb shoulder, pressing into the scored cuts.
05 - Place sliced onions, carrots, and celery in base of a large roasting pan. Position lamb atop vegetables.
06 - Pour broth around lamb without pouring over it.
07 - Cover pan tightly with foil and roast 2.5 hours.
08 - Remove foil, increase oven to 400°F, roast 30 minutes more, basting occasionally until golden and tender.
09 - Transfer lamb to platter, loosely cover with foil, rest 15 minutes before carving.
10 - Carve lamb and serve with roasted vegetables and pan juices.