Heat the oil in a large pot on medium heat. Add in the chopped carrots, celery, onion and garlic and sauté for approximately 5 minutes until the vegetables begin to soften.
Add the cans of tomato, vegetable broth, thyme, oregano, red pepper flakes and lentils and bring to a boil.
Reduce the heat to low, cover the pot and leave to simmer for 1 hour.
After 1 hour remove the lid and continue to cook for approximately 45 minutes until the sauce has reduced by half and the lentils have become very tender.
Serve the sauce on its own or on top of pasta, zucchini noodles or spaghetti squash. This sauce freezes very well.