Spray the inside of a 6-quart slow cooker with cooking spray.
Combine thawed hashbrowns, cream of chicken soup, sour cream, 1 cup cheddar, milk, and diced onion directly in the slow cooker. Stir until evenly mixed.
Dot the top with butter, or melt the butter and stir it in.
Cover and cook on high for 2 hours without lifting the lid.
Sprinkle remaining 1/2 cup cheese over the top. Replace lid for 5 minutes, until cheese melts.
Serve straight from the slow cooker.
Notes
Thaw the hashbrowns first so extra water does not thin the sauce. Use two cans of cream of chicken soup for the creamiest texture. Reserve some cheese for the top so the finished potatoes have a fresh melted layer. Avoid opening the lid during cooking because it adds time.