01 - Generously grease the interior of the crockpot insert with butter to prevent sticking.
02 - Melt 2 tablespoons butter in a small saucepan over medium heat. Add minced garlic and sauté until fragrant, about 1 minute. Stir in flour and cook for 1 additional minute to form a roux. Gradually whisk in heavy cream and whole milk until smooth. Bring to a gentle simmer, then remove from heat. Stir in salt, pepper, and half the cheddar cheese until melted and combined. Set aside.
03 - Arrange half the sliced potatoes in the bottom of the prepared crockpot, overlapping slices as necessary. Distribute half the chopped onion evenly over potatoes, followed by half of the remaining cheddar cheese. Pour half the cheese sauce over the first layer.
04 - Create a second layer with remaining potato slices, chopped onion, and cheddar cheese. Pour the remaining cheese sauce over the top. Sprinkle with paprika if using.
05 - Cover and cook on high heat for 4 hours or low heat for 7 hours, until potatoes are completely tender when pierced with a fork.
06 - Remove the lid and allow the dish to rest for 10 minutes before serving. This helps the sauce thicken slightly. Garnish with chopped fresh parsley if desired.