Heat the vegetable oil in a large pan and add the onion, celery and a pinch of salt. Put on a lid and cook gently for 10 minutes, stirring regularly. Add the garlic, preserved lemons and red chillies, and cook for 2 minutes. Add the tomato purée and spices, and cook for a further 2 minutes before adding the chopped tomatoes, potato, chickpeas and 1.25 litres of boiling water.