In a saucepan on low heat, cook the onion and the cumin seeds in oil. Once fragrant add the minced garlic and cook while stir for a few seconds. Add the lentils and carrots and continue to cook and stir for a couple of minutes.
Add to the pan the passata, turmeric and bay leaf. Cook for a few minutes, stirring often. Pour in the water or stock, adjust seasoning with salt, pepper and chilli (if using). Cover and cook for 15 to 20 minutes or until the lentils are tender.
Once ready, serve the lentil soup as is or blend with a stick blender or food processor. Ladle into warm bowls and serve.