Prepare the sauce
In a mixing bowl, whisk together the heavy cream, milk, melted butter, flour, minced garlic, salt, and black pepper until smooth.
Layer the ingredients
Grease the inside of the crock pot with cooking spray. Arrange a layer of sliced potatoes at the bottom, followed by a layer of onions. Sprinkle with cheddar cheese and mozzarella cheese. Repeat the layers until all ingredients are used, finishing with cheese on top.
Pour the sauce
Evenly pour the prepared cream sauce over the layered potatoes and cheese, ensuring it seeps down to the bottom.
Cook the potatoes
Cover and cook on low for 6-7 hours or on high for 3-4 hours, until the potatoes are tender and easily pierced with a fork.
Serve and garnish
Once cooked, sprinkle with paprika and fresh parsley for garnish (optional). Serve warm and enjoy!