Spray, butter or oil the slow cooker insert.
Wash potatoes, remove bad spots and slice 1/4 inch quick. Set aside.
Mix melted butter, soup and cheeses in crockpot. Stir well.
Add onions and stir in.
Pour in potatoes and stir til potatoes are well coated.
Cover and cook on low for 5 to 7 hours, or til potatoes are tender.