Heat oil in a large stock pot on medium heat. Add onion, potatoes, garlic, mushrooms, salt and pepper. Cook until onion is soft.
Add water, lentils and the remaining spices. Trim leaves of collard greens, discarding the stems. Cut greens into thin pieces and add to soup. Bring to a boil, reduce heat to medium low, cover and let simmer for 45-60 minutes, or until lentils are cooked to your liking.
Serve hot. Serves 6-8