Cut the lamb into chunks and pat dry. Heat half the olive oil in a braising pan and brown the meat in batches all over. Don't put too much in the pan at once, otherwise it stews instead of browning. Remove the meat and set aside.
Cut the aubergine into chunks and the onion into half rings. Fry them in the remaining oil until the aubergine is golden brown and the moisture has evaporated. Add the finely chopped garlic and the ras el hanout and fry for a minute until the spices are fragrant.
Return the meat to the pan, pour in the passata with a splash of water and season with salt and pepper. Let the stew simmer gently with the lid slightly ajar for at least ninety minutes until the lamb falls apart. Stir through the chickpeas in the last ten minutes. Finish with roughly chopped coriander. Slow cooking softens the connective tissue and creates a full, spicy sauce.