Heat the oil in a large pot over medium heat. Add the onion, carrots, and celery. Add the garlic, salt, and pepper and sauté until all the vegetables are tender, about 8 minutes. Add the tomatoes with their juices. Simmer until the juices evaporate a little and the tomatoes break down, stirring occasionally, about 8 minutes. Add the lentils and mix to coat. Add the broth and stir. Bring to a boil over high heat. Cover and simmer over low heat until the lentils are almost tender, about 30 minutes. Sprinkle with the Parmesan.
Another alternative is to follow instructions as above, but use less broth and serve as a skillet. A tasty addition is sprinkling crumbled bacon bits on top