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. Cook for 5 mins, stirring now and then, till softened.
While the onions cook, peel and finely chop or grate the carrot. Very thinly slice the celery. Peel and chop your garlic. Add to the pan, along with 2 tsp cumin seeds. Sizzle over medium-low heat for 5 mins.
Add the chopped tomatoes. Fill the empty tomato tin with warm water (around 400 ml). Add it to the pan. Cover and bring to the boil. Reduce the heat. Simmer for 5 mins.
Drain the chickpeas and add to the pan. Simmer for 5 mins. Grate the zest from the lemon. Squeeze in a little lemon juice. Taste and add more salt, pepper or lemon juice if it needs it. Ladle the soup into bowls and top with lemon zest, parsley leaves and any leftover cumin seeds to serve.