01 - Set the oven temperature to 325°F.
02 - Combine garlic, rosemary, thyme, parsley, oregano, lemon zest, lemon juice, kosher salt, black pepper, and olive oil in a bowl to create a paste.
03 - Pat the lamb shoulder dry, then score the fat in a criss-cross pattern using a sharp knife. Rub the herb paste evenly over the entire lamb, pressing into the scores.
04 - Place onions, carrots, and celery in the bottom of a large roasting pan.
05 - Set the lamb shoulder on top of the vegetables. Pour the broth around the lamb without pouring over it.
06 - Cover the roasting pan tightly with foil and roast for 2.5 hours.
07 - Remove the foil, increase oven temperature to 400°F, and roast for an additional 30 minutes, basting occasionally, until the meat is golden and tender.
08 - Transfer the lamb to a platter, loosely cover with foil, and let rest for 15 minutes before carving.
09 - Carve and serve the lamb with roasted vegetables and pan juices.