Heat oil in a saucepan over medium. Sauté onion, carrots, and celery for 7-8 minutes, stirring often. Add garlic, and continue to sauté 20-30 seconds. Add tomato paste and cook for 1 minute. Stir in broth or water, diced tomatoes, and lentils. Season with cumin, basil, oregano, thyme, salt, and red pepper flakes. Bring mixture to a boil; reduce heat and simmer gently for 15-20 minutes (depending on your stove), or until lentils are soft but not mushy.