01 - Preheat the oven to 320°F.
02 - In a small bowl, combine minced garlic, rosemary, thyme, lemon zest, parsley, salt, pepper, olive oil, Dijon mustard, and lemon juice. Mix thoroughly to form a cohesive paste.
03 - Pat the lamb shoulder completely dry with paper towels. Rub the herb paste generously over all surfaces, working the mixture into any crevices and under the fat layer.
04 - Arrange the sliced onions, carrots, and celery chunks evenly across the bottom of a large roasting pan to create a bed for the meat.
05 - Place the seasoned lamb shoulder directly on top of the vegetable bed, fat-side up.
06 - Pour the white wine or chicken broth around the base of the lamb, avoiding pouring directly over the seasoned meat.
07 - Cover the roasting pan tightly with aluminum foil or a lid. Roast for 2.5 hours, uncovering briefly every 45 minutes to baste the lamb with accumulated pan juices.
08 - Remove the foil and increase the oven temperature to 430°F. Return the lamb to the oven and roast uncovered for 30 minutes until the exterior develops a deep golden-brown, crispy crust.
09 - Transfer the roasted lamb to a serving platter and allow it to rest for 15 minutes before carving. Meanwhile, strain the pan juices through a fine mesh sieve and serve alongside the sliced meat.