Preheat the oven to 220ºC. Rub sumac and garlic cloves over the lamb, brush the lamb with a little oil, sprinkle with thyme leaves and season with salt and pepper.
Place the lamb on a rack in a roasting dish. Roast for 15 minutes for rare, 20-25 minutes for medium, or 25-30 minutes for well done.
Remove lamb, cover loosely, rest lamb for 10 minutes before serving.
To make salad combine the oil, lemon rind and juice, salt and sugar to form a dressing. Make up the instant couscous according to directions on packet. Combine couscous with chickpeas, cucumber, capsicum and coriander leaves and pour over dressing.
Serve lamb sliced on top of a spoonful of salad with a squeeze of lemon.