01 - Set oven temperature to 325°F (160°C).
02 - Combine garlic, rosemary, thyme, parsley, oregano, lemon zest, lemon juice, salt, pepper, and olive oil in a bowl to form a paste.
03 - Pat lamb shoulder dry, score fat in a criss-cross pattern, and rub the herb paste all over, pressing into the cuts.
04 - Place onions, carrots, and celery in the bottom of a large roasting pan.
05 - Place lamb shoulder on top of the vegetables and pour broth around the lamb, avoiding pouring over it.
06 - Cover pan tightly with foil and roast for 2.5 hours.
07 - Remove foil, increase oven temperature to 400°F (200°C), and roast for an additional 30 minutes, basting occasionally until lamb is golden and tender.
08 - Transfer lamb to a platter, loosely cover with foil, and let rest for 15 minutes.
09 - Slice lamb and serve with roasted vegetables and pan juices.