Heat oil in large heavy pot over medium heat. Add onion, carrots, and celery. Add garlic, salt, and pepper and sauté until all vegetables are tender (~5-8 minutes). Add tomatoes with their juices. Simmer until juices evaporate a little and tomatoes begin to break down, stirring occasionally (~8 minutes). Add lentils and mix to coat. Add broth and stir. Add thyme sprigs. Bring to a boil over high heat. Cover and simmer over low heat until the lentils are almost tender (~30 minutes).