Pour the dried chickpeas into a 30cm x 40cm roasting tray. Quarter the preserved lemons and trim away the seedy core, then finely chop the rind and add to the tray with a good splash of liquor from their jar. Roughly chop the tomatoes, adding them to the tray as you go.
Drizzle the lamb with 1 tablespoon of olive oil, then rub all over with the ras el hanout and a pinch of sea salt and black pepper. Sit the lamb in the tray, pour in 1 litre of water, cover tightly with tin foil and place in a cold oven. Turn the temperature to 170ºC/325ºF/gas 3 and leave the lamb in there for 6 hours, or until the chickpeas are cooked through and the lamb is pullable – after 3 hours, stir a splash of water into the chickpeas, covering tightly again with foil.
To serve, taste the chickpeas, season to perfection, and drizzle with 1 tablespoon of extra virgin olive oil, then pull the lamb apart with two forks.