Grease the inside of your slow cooker with butter or cooking spray.
Layer half of the sliced potatoes and onions in the slow cooker.
In a mixing bowl, combine cream of mushroom soup, milk, and a pinch of salt and pepper. Pour half of this mixture over the potatoes and onions.
Sprinkle half of the cheddar and Parmesan cheese over the top.
Repeat with the remaining potatoes, onions, soup mixture, and cheeses.
Top with small pieces of butter.
Cook on low heat for 6-8 hours or on high for 3-4 hours until potatoes are tender and the cheese is bubbly.
Serve warm and enjoy!