Put all the vegetables in a pot with about an inch/2cm of cold water. Stir together, then cook over a gentle heat, with the lid on, for about 15 minutes.
Add the rinsed red lentils, the stock and the tomatoes. Stir, bring to a boil, then turn down to a simmer.
Keep the lid on and cook for a further 20 minutes. Keep an eye on the soup and stir occasionally to ensure it doesn't catch-on. Add more hot water if you think it's too thick.
Heat some bowls then serve the soup for lunch or as a starter with some homemade wholemeal bread.