01 - Preheat the oven to 320°F.
02 - Pat the lamb shoulder dry with paper towels and score the fatty side in a diamond pattern using a sharp knife. Rub the entire surface with olive oil, then season generously with salt and pepper.
03 - In a small bowl, combine the minced garlic, chopped rosemary, thyme leaves, oregano, lemon zest, and lemon juice. Mix thoroughly to form a thick paste.
04 - Massage the herb paste evenly over the lamb shoulder, working the mixture into the scored cuts to maximize flavor penetration.
05 - Arrange the sliced onions, carrot chunks, and celery pieces in the bottom of a large roasting pan to create an aromatic vegetable bed. Place the seasoned lamb shoulder on top, fatty side facing up.
06 - Pour the 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.
07 - Roast the covered lamb for 2.5 hours at 320°F until the meat begins to tenderize.
08 - Remove the foil and increase the oven temperature to 400°F. Continue roasting uncovered for 30 minutes until the surface develops a deep golden-brown crust and the meat is fork-tender.
09 - Transfer the lamb to a cutting board and let it rest, loosely tented with foil, for 15 minutes to allow juices to redistribute throughout the meat.
10 - Carve the lamb against the grain and serve with the roasted vegetables from the pan, spooning the warmed pan juices over each portion.