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.
Bring to the boil, then turn down and simmer for 30 minutes until the potatoes are tender. Tip in the spinach, flat-leaf parsley and a little seasoning, and stir for 1 minute until wilted. Add the lemon juice, then spoon into bowls with lemon wedges for squeezing.