1.Preheat oven to 220°C. To make shawarma spice rub, pound garlic and 2 tsp sea salt with a mortar and pestle to a paste, add preserved lemon rind, baharat, Aleppo pepper and sumac and pound to a paste (you can also do this in a small food processor). Stir in oil and season to taste. Rub half spice paste all over lamb rack.
2.Place lamb rack in a roasting pan and roast until cooked to your liking (20-25 minutes for medium). Rest for 5 minutes before carving to serve.
3.Meanwhile, to make couscous, place stock in a saucepan and bring to the boil (3-5 minutes). Add couscous and reduce heat to medium. Simmer until couscous is three-quarters cooked (15-20 minutes); remove from heat, cover with foil, and leave to stand until cooked through. Add remaining ingredients, season to taste and stir to combine.
4.Divide couscous between plates and top with carved lamb rack. Serve with labne, extra marinade, Aleppo pepper, oil and lemon wedges.
Baharat is a Middle Eastern spice blend available from herbies.com.au or Middle Eastern delicatessens. Aleppo pepper, a ground mild chilli, is available from Turkish grocers. If unavailable, substitute roasted chilli flakes.