**Prepare the Potatoes:** Peel and thinly slice the potatoes using a mandoline slicer or knife, ensuring even thickness for uniform cooking.
**Mix the Sauce:** In a mixing bowl, whisk together heavy cream, milk, melted butter, flour, garlic powder, onion powder, paprika, salt, and pepper until smooth.
**Layer the Potatoes and Cheese:** Lightly grease the slow cooker with non-stick spray or butter. Layer half of the potato slices in the bottom of the slow cooker, sprinkle with half the cheddar and mozzarella cheese, and pour half the sauce over the top. Repeat with the remaining potatoes, cheese, and sauce.
**Cook:** Cover and cook on LOW for 6–7 hours or HIGH for 3–4 hours, until the potatoes are tender and the cheese is bubbly.
**Garnish and Serve:** Sprinkle with fresh parsley before serving for a pop of color and added flavor.