1. Into a large saucepan heat olive oil over a medium heat. Add in celery, carrots, onions and garlic. Saute this until the vegetables become brown which should take about 15 minutes.
2. Add in 4 cups of broth, tomatoes with the juice and lentils, bringing this to the boil. Reduce this to a medium-low heat, cover this and then simmer for 35 minutes by which time the lentils are tender.
3. Transfer 2 cups of soup to the blender and then puree this until smooth. Return this puree to the pan. For a thin soup, add 1/4 cup of broth and add more if it’s too thick. Season this with salt, pepper and a splash of balsamic vinegar if necessary.
4. Ladle the soup into bowls and then garnish with celery leaves.