1. Heat a splash of oil and add the onions and a pinch of salt, cook until softening
2. Add the chickpeas, lentils, tomatoes and celery
3. Add the tomato puree and spices, cook for 5 minutes
4. Next add the stock
5. When the stock is hot through add 2 cupfuls to the sourdough starter
6. When combined slowly add back into the soup
7. When the vegetables and lentils are softened whizz the soup, add water to reach the desired consistency
8. Add the chopped herbs and season to taste