Place the lamb in a roasting tray, drizzle with olive oil and rub the spices and salt into the meat. Cover with cling wrap and marinate in the fridge overnight.
Next day, preheat the oven to 140 degrees C.
Remove the cling wrap and add 1 cup water to the roasting tray, then cover with foil to seal. Roast in the oven 2 hours, then turn the temperature to 110 degrees C. Cook a further 5 hours, removing the foil for the last hour to crisp the skin. During cooking, check every now and then to make sure the pan is not completely dry - add a little water if necessary.
When the lamb is cooked, roughly shred the meat and spread on a platter. Place dollops of yoghurt around it and sprinkle with torn mint, shallot slices and pomegranate seeds.
Serve with the harissa, lemon quarters and extra salt, if needed.