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