In a large saucepan over medium heat, melt butter. Add in flour and cook for a minute to brown. Pour in milk, cheddar cheese, parmesan cheese, salt, and pepper. Continue to cook over medium heat until smooth and thickened.
Spread half of the potatoes out into a greased crock-pot. Top with half of the cheese sauce. Add the remaining half of the potatoes on top and cover with the remaining sauce. Sprinkle with parsley. Cover and cook on high for 3-4 hours.