Lightly grease the inside of your slow cooker with nonstick spray.
Layer half of the sliced potatoes evenly in the slow cooker.
Sprinkle half of the chopped onion and half of the cheese over the potatoes.
Repeat with the remaining potatoes, onion, and cheese.
In a bowl, whisk together the condensed soup, milk, melted butter, garlic powder, salt, and pepper until smooth.
Pour the mixture evenly over the layered potatoes.
Cover and cook on LOW for 6–7 hours or until potatoes are tender.
Garnish with chopped parsley before serving, if desired.