| • | Heat the oil in a heavy saucepan over medium-high heat.
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| • | Add chopped onions, celery, carrots, and garlic; sauté until vegetables begin to brown, or for about 15 minutes.
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| • | Add vegetable broth, lentils and tomatoes with juice; bring to a boil.
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| • | Reduce heat to medium-low. Cover the pan and simmer until lentils are tender, or for about 35 minutes.
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| • | Transfer 2 cups of this mixture (mostly solids) to blender and puree until smooth.
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| • | Return puree to the soup in the pan. If the soup is too thick, then add about 1/4 cup of broth to it.
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| • | Season the soup with salt, pepper and a splash of vinegar, if desired.
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| • | Ladle soup into bowls. Garnish with chopped celery leaves. |
