Remove the bone from the lamb, make a horizontal cut through the thickest part of the meat then open out so it is about 4 cm (1½ inch) thick. Place in a shallow non-metallic dish. Cut the pomegranate in half and scoop out the seeds. Place half the seeds in a pestle and mortar with the garlic then crush roughly. Mix with half the mint, half the oil, the honey and half the lemon rind and juice. Season and spoon over the lamb. Cover and chill for at least 3 hours.
Put the couscous into a microwave-proof serving bowl, add the dried fruit, onion, remaining lemon rind and juice, then pour over the boiling water. Allow to cool then cover with clingfilm and chill until required.
When ready to serve, transfer the lamb to a roasting tin with some of the marinade. Roast in a preheated oven, 220°C (425°F), Gas Mark 7, for 20–25 minutes for medium rare, or 30–35 minutes for well done. Lift out the lamb, add the remaining marinade to the hot pan and mix with the pan juices. Pour over the lamb, cover tightly with foil and leave to stand for 10 minutes.
Add the reserved pomegranate seeds and pistachios to the couscous. Mix the remaining oil with the harissa, if using, drizzle over the couscous then replace the clingfilm and microwave on full power for 1½–2 minutes until piping hot. Sprinkle the remaining mint over the top and fluff up with a fork. Spoon on to serving plates, top with thickly sliced lamb and drizzle with pan juices.