Heat the oil in a heavy large pot over medium heat. Add the onion, carrots, and celery. Add the garlic, salt, and pepper and saute until all the vegetables are tender, about 5 to 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. Add the thyme sprigs. Bring to a boil over high heat. Cover and simmer over low heat until the lentils are almost tender, about 30 minutes.