Heat the oil in a large saucepan and then gently fry the onion, garlic, celery and carrot until the vegetables start to soften.
Add the tomatoes, stock, and lentils, tomato purée and the herbs. Stir to mix and then bring to the boil. Reduce the heat and simmer for 15 - 20 minutes until the lentils are soft. Season with salt and pepper and add the nutritional yeast.
Remove from the heat and blend until smooth with a stick blender.