Place the lentils in a bowl of water. Heat the oil in a large saucepan and gently fry the onions, garlic and celery over a low heat until softened.
Drain the lentils and add them to the vegetable pan with the tomatoes. Mix well. Add the chilli, if using, paprika, harissa paste, cumin and vegetable stock and season with salt and pepper. Cover the pan and simmer gently for about 40–50 minutes until the lentils are tender, adding a little more vegetable stock or water if the soup gets too thick.
Serve the soup immediately in warmed individual bowls topped with a little chopped coriander.