Spray a large saucepan with the cooking spray and cook the onion on a medium heat until it begins to soften. You don't need to be too precise about the chopping, as it's all going in the blender later.
Add the carrot and celery and continue to fry for a few minutes until starting to soften.
Then add all the other ingredients.
Bring to the boil, cover and simmer for about 45 minutes, until the lentils are soft. Cool slightly then blend in batches and serve straight away.