01 - Preheat the oven to 320°F for slow roasting.
02 - Combine garlic, rosemary, thyme, lemon zest, parsley, salt, pepper, olive oil, Dijon mustard, and lemon juice in a small bowl. 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 crevices and under any fat layers.
04 - Arrange onions, carrots, and celery in an even layer at the bottom of a large roasting pan. Place the seasoned lamb shoulder directly on top of the vegetable bed.
05 - Pour white wine or chicken broth around the base of the lamb, avoiding direct pouring over the meat. Cover the roasting pan tightly with aluminum foil or secure the lid.
06 - Roast covered for 2.5 hours. Open the pan every 45 minutes to baste the lamb with accumulated pan juices, then recover and continue cooking.
07 - Remove the foil and increase 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.
08 - Transfer the lamb to a serving platter. Let rest for 15 minutes before carving to allow juices to redistribute. Strain the pan juices through a fine mesh sieve and serve alongside the sliced meat.