First make the spice paste. Put the chopped onion into a food processor along with the chopped garlic and ginger. Pulse until finely minced, then add the parsley and spices. Pulse for about 10 seconds, until just combined. Scrape down the sides, then add the vinegar, oil, 2¼ teaspoons of salt and a generous grind of black pepper. Pulse to form a coarse paste, then transfer to a non-metallic container large enough to hold the lamb. Pat the lamb dry and pierce liberally all over with a small, sharp knife. Add it to the spice paste and coat generously, so that all sides are covered. Cover with foil and leave to marinate, refrigerated, overnight. Take the lamb out of the fridge about an hour before going into the oven: you want it to be more like room temperature rather than fridge-cold. Preheat the oven to 140°C fan. Put the remaining onions and head of garlic into the centre of a large roasting tray and pour over the chicken stock. Sit the lamb on top of the vegetables, cover tightly with foil and bake for 4 hours. Remove from the oven, discard the foil and bake for 90 minutes more, increasing the oven temperature to 160°C fan towards the last 30 minutes of cooking time. The lamb is ready when it is fork-tender and easily pulls away from the bone. Set aside to cool slightly, about 15 minutes, before using two forks to roughly shred the lamb directly in the pan, gathering as much of its juices as possible. Transfer the shredded lamb, onions, garlic cloves and any of the pan juices to a serving bowl. Squeeze over the lemon juice and set aside. While the lamb is in the oven, prepare the sumac yoghurt. Put the yoghurt, tahini, lemon juice, 2 tablespoons of water, the sumac and ¼ teaspoon of salt into a bowl and whisk well to combine. When ready to serve, lay out all the various condiments, along with the pita, to let everyone make up their own shawarma sandwich.