Place first 8 ingredients in a 5- or 6-qt. slow cooker. Add cabbage. Cook, covered, on low until lentils are tender, 6-8 hours., Stir in mushrooms and salt. Cook, covered, on low until mushrooms are tender, 15-20 minutes. Freeze option: Freeze cooled soup in freezer containers. To use, partially thaw in refrigerator overnight. Heat through in a saucepan, stirring occasionally; add water if necessary.