Open and Score: Lay the 7.5 lb boneless lamb shoulder flat on your cutting board, opening out its natural folds and pockets without twine. Pat the meat thoroughly dry with paper towels. Score the exterior fat cap in a tight diamond pattern, cutting just through the fat layer.

Mix the Marinade: Combine the extra-virgin olive oil, minced garlic, cumin, paprika, ginger, coriander, turmeric, cayenne, kosher salt, black pepper, cilantro, parsley, lemon juice, and minced preserved lemon (or date syrup) in a bowl. Whisk into a vibrant, aromatic paste.

Massage the Folds: Rub the marinade generously over every exposed surface, pulling back the natural seams of the meat to work the spices deep into the interior layers and the diamond-scored fat. Leave it laid out open or loosely folded naturally (no tying required). Cover and refrigerate for 12 to 24 hours.

Build the Roasting Bed: Preheat your oven to 300°F (150°C). Lay the thick-sliced onions and halved garlic head across the base of a heavy roasting pan, and pour the vegetable stock around them. Place the marinated lamb directly on top of the onions, fat-side up.

Slow Roast (Covered): Seal the roasting pan tightly with a double layer of heavy-duty aluminum foil to lock in moisture. Place in the oven and roast for 3.5 to 4 hours until the meat is entirely tender.

Develop the Bark (Uncovered): Remove the foil. Turn the oven up to 400°F (200°C). Roast uncovered for 45 to 60 minutes, basting once with the pan liquids halfway through, allowing the exposed interior seams and natural fat cap to render and form a deep, crusty bark.

Rest and Serve: Transfer the lamb to a cutting board, tent loosely with foil, and rest for 25 to 30 minutes. Slice or pull into thick portions, serving atop the caramelized onions from the pan.