1. In a medium saucepan, sauté the onions, celery, carrots, and garlic over medium-high heat for 12 to 15 minutes.
2. Once the vegetables have just started to turn light brown, add the salt, dried thyme, and black pepper to the pan. Continue cooking and stirring the mixture for 1 minute.
3. Add the chicken or vegetable stock, lentils, and diced tomatoes to the pan. Bring the soup to a boil, and then reduce the temperature slightly so that it is simmering. Simmer the soup, covered, for 30 minutes, until the lentils are tender.
4. Puree 1 3/4 cups of the soup with the cider vinegar and stir the pureed mixture back into the soup.
5. Serve the soup hot or store it for up to three days in the refrigerator.