In a heavy soup pot, heat the olive oil on medium heat. Add the onions, carrots, celery and garlic. Cook, stirring occasionally, for 10 minutes or until the vegetables start to brown a little.
Add the vegetable stock or add the water and stir in the bouillon cubes, making sure they dissolve completely. Add the lentils, canned tomatoes, and bouquet garni. Bring to a boil, then turn the heat down so the soup is just barely boiling.
Cover partly with a lid and then cook for at least 35 minutes or until the lentils are tender. You may wish to add some more water if the soup becomes too thick for your taste. Towards the end of cooking, stir in the champagne vinegar and season to taste with salt and freshly ground pepper.
Makes 6 servings.