Heat the oil in a large saucepan and add the onion, celery and garlic. Cook over a low heat for 5-8 minutes until soft but not browned. Stir in the cauliflower and chickpeas, add the harissa and cumin, then stir to coat the vegetables in the spices.
Add the preserved lemons, tomato purée, stock and seasoning. Bring to the boil and simmer gently, covered, for 20 minutes until the vegetables are tender. Stir in the lemon juice, scatter with the coriander and a little sumac, then serve.
