1) Using a large pan, sauté onions, garlic, mushrooms in olive oil until onions are translucent. Add tomato paste and cook for another two minutes while stirring.
2) Add stock and lentils. Simmer with the lid on for an hour and a half or until lentils are tender.
3) Add carrots and tomatoes or salsa and salt and pepper to taste.
4) Add avocado to individual bowls just prior to serving and top soup with shredded cheese if desired (optional).