2.Put the sumac, preserved lemon, garlic, coriander and salt into a small bowl and stir to combine. Season with freshly ground black pepper.
3.Rub the spice paste over the lamb and then place it into a large oven bag. Tie bag following packet directions. Transfer to a baking dish. Roast lamb for 2 1/2 hours. Set lamb aside to rest in the oven bag for 20 minutes.
4.Combine oil and pomegranate molasses in a small bowl; season to taste.
5.Remove lamb from oven bag and transfer to a large serving plate. Using two forks, roughly shred the meat, then add the herbs, walnuts and pomegranate seeds. Drizzle with the dressing and sprinkle with the feta.