Peel and thinly slice the onions. Warm a large pan for 1 min. Add 1 tbsp oil and the onions. Season with salt and pepper. Keep the heat low and cook for 5 mins, stirring now and then, till softened.
While the onions cook, peel and finely chop or grate the carrot. Trim and very thinly slice the celery. Peel and chop the garlic. Add them all to the pan, along with 2 tsp cumin seeds. Sizzle over a medium-low heat for 5 mins.
Add the chopped tomatoes. Fill the empty tomato tin with warm water (around 400ml). Add it to the pan. Cover and bring to the boil. Reduce the heat. Simmer for 5 mins.
Drain the chickpeas and add them to the pan. Simmer for 5 mins. Grate the zest from the citron beldi lemon and set aside. Squeeze in a little of the juice – it has a strong, floral flavour so add a small squeeze to start with. Taste and add more salt, pepper or citron beldi juice if it needs it.
Ladle the soup into warm bowls and top with the citron beldi zest and parsley leaves to serve.