Heat a large saucepan, spray with low fat cooking spray and add the onion and celery. Cook stirring for 5 mins, add the garlic and chili and cook for another 5 mins until tender and beginning to brown.
Add the lentils, stock, rosemary, tomatoes and tomato paste. Bring to the boil, reduce the heat and simmer for 20 mins until the lentils are soft. Remove the rosemary.
Blend the soup using a hand blender or liquidizer until smooth and season with salt and pepper.
Serve hot garnished with a sprinkling of chopped rosemary.