Spray inside of the slow cooker with cooking spray. Add potatoes (I like to use the shredded hash browns vs. the cubed), soup, milk and onion rings (reserving 1/4 cup of the onion rings). Add salt and pepper to taste.
Stir a little to combine. Cover and cook on LOW for 7-9 hours or HIGH about 3 hours. Sprinkle reserved onion rings over the top just before serving.