Preheat your oven to 160°C (325°F). Pat the Bone-In Lamb Shoulder dry with paper towels and trim any excessively long hanging fat, then score the fat and skin in a shallow diamond pattern.
Warm about 60–100 g of Smen gently in a small saucepan or microwave until spreadable, then stir in 2–3 tablespoons of Ground Cumin and 1–2 teaspoons of Coarse Salt to make a fragrant paste.
Work the spiced Smen paste all over the lamb, pushing some into the scoring and underneath the skin where possible so the flavors penetrate the meat.
Place the shoulder on a rack set over a roasting pan. Pour about 250–500 ml of Water into the bottom of the pan to keep the environment moist and catch drippings for basting.
Cover the pan tightly with foil and roast in the preheated oven for 3–4 hours (depending on size) until the meat is very tender and pulls away from the bone; baste every hour with the pan juices.
Remove the foil, increase the oven temperature to 220°C (425°F), and roast uncovered for 20–30 minutes more to develop a deeply browned, slightly crisp exterior.
Transfer the shoulder to a cutting board, tent loosely with foil, and rest for 20–30 minutes so the juices redistribute.
Carve the Mechoui into thick slices or pull the meat from the bone and serve warm, spooning any pan juices over the meat.
