Put the lentils, rice, chickpeas and vegetables in a large pot. Add the stock, water, tomato puree and spices. Cover and bring to the boil, then reduce heat and simmer 45 minutes or until lentils have pulped and thickened the soup.
Remove the chillies, adjust the seasoning, and swirl a little dollop of yoghurt through each bowl when serving. Sprinkle with chopped mint.