Preheat oven to 150C. Heat oil in a large casserole over high heat, add lamb neck in batches and sauté, turning occasionally, until golden (4-5 minutes each side). Remove from pan and set aside.
Add onion and garlic to pan, stir occasionally over low-medium heat until caramelised (10-12 minutes). Add celery, sauté until just beginning to soften (3-4 minutes), then add spices and stir until toasted and fragrant (1 minute). Return lamb neck to pan, add olives, preserved lemon and 500ml water, cover and cook in oven until meat falls from the bone (2½ hours). Stir through coriander just before serving.
Meanwhile, place couscous in a heatproof bowl, pour in 500ml boiling water, add butter, cover bowl tightly with plastic wrap and stand for 10 minutes. Remove plastic wrap, season generously to taste, fluff couscous with a fork, transfer to serving platter, top with lamb and serve hot.