Add potatoes, cabbage, onions, salt and water to crock pot.
Cook 7 hours on low.
After cooking is complete, drain remaining water.
Mash potatoes and cabbage with hand blender.
Add to serving dish and drizzle with butter and sprinkle with pepper.
Notes
1. I used an Instant Pot on the slow cooker setting. I find the Instant Pot is the best tool for cooking a variety of foods. Check out my Review of Instant Pot 6 Quart Pressure Cooker here. 2. If you don't have a crock pot or Instant Pot, you can cook this dish on the stove top. Add all ingredients to a large soup pot and add water to top. Bring to boil, and reduce to simmer for 2 hours, or until potatoes are cooked. Drain and blend as described above. 3. If you prefer, you can cook this dish in the Instant Pot. Add all ingredients to Instant Pot, and cook on high pressure for 8 minutes. Drain and blend as described above.