Pat dry the lamb and season with salt. In a small bowl, combine the black pepper powder, white pepper powder, cinnamon powder, 7-spice powder, turmeric powder, orange zest and olive oil to make a smooth marinade paste, and generously rub it all over the lamb.
Preheat the oven to 180°C.
In a large cast iron pot, add a couple of tablespoons of vegetable oil. Heat on medium fire and sear the lamb on all sides.
Remove the lamb and reserve on the side. In the same pot, add onions, carrots, celery, garlic bulbs, orange slices, thyme sprigs and bay leaves. Add the seared lamb and cover with chicken stock and water, and season with salt. Cover the pot with the lid and place it in the oven pan, and roast for 4 hours. Transfer the lamb to a large plate, cover with foil and allow to rest for 20 minutes. Shred the meat with two forks.
To prepare the tahini sauce, mix all the ingredients until it has a smooth consistency.
To prepare the marinated onions, in a small bowl mix onions with a pinch of salt. Add the olive oil and sumac. Allow to rest for 10 minutes before using.
To prepare the wrap, arrange rocket leaves in the base of the bread. Top with slices of tomato, pulled lamb, tahini sauce, and marinated onions. Serve as an open sandwich or as a wrap.