Rinse the lamb and pat dry with kitchen paper. Tip into a large pan and add tomatoes. Half fill the tomato can with water and add to the pan with the harissa paste and a good sprinkling of salt and pepper.
Bring the liquid to the boil, then reduce the heat, cover and simmer for 1-11⁄4 hours, until the lamb is tender. Rinse the chickpeas and add them to the pan, then simmer for a further 5 minutes.
Taste and add more seasoning if necessary. Roughly chop the coriander, then scatter over the dish. Serve with couscous or rice.