Heat the olive oil in a large soup pot over medium-high until hot. Add the carrot, celery, onion, and garlic, and saute for 4 minutes, or until vegetables have begun to soften.
Stir in the lentils, water, vegetable broth, and tomatoes. Bring to a boil.
Reduce heat and simmer for 1 hour, until lentils are cooked and the broth has reduced.