Add olive oil to a 6-to-7-quart soup pot on medium heat. When oil is hot, add onions, carrots, celery, and garlic and sauté on medium heat until very soft and fragrant, about 15 minutes. Add zucchini and mushrooms and continue cooking for another 5 minutes. Add the juice from tomatoes and chop the tomatoes before adding to soup. Simmer the vegetables and tomatoes for 10 minutes. Add the lentils and combine with cooked veggies. Add water leaving the level at about three inches from the lentils. The lentils will swell, break down, and continue to require more water through the cooking. Stir frequently and allow to simmer for another 35 minutes, adding water if necessary. The final consistency should be like that of pea soup.