Grease the inside of the Crock Pot with cooking spray or a little butter to prevent sticking.
Layer half of the sliced potatoes in the bottom of the Crock Pot. Top with half of the sliced onions and half of the shredded cheese.
In a medium bowl, whisk together the heavy cream, milk, melted butter, garlic powder, salt, black pepper, paprika, and cayenne pepper (if using).
Pour half of the cream mixture over the first layer of potatoes, onions, and cheese.
Repeat the layers with the remaining potatoes, onions, and cheese, then pour the rest of the cream mixture on top.
Cover and cook on low for 6 to 8 hours or on high for 3 to 4 hours, until the potatoes are tender and the cheese is bubbly.
Once done, let it sit for about 10 minutes before serving. Garnish with chopped parsley.