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. Sprinkle in flour and stir constantly for 1 minute to cook out the raw taste. Gradually whisk in heavy cream and whole milk, ensuring no lumps form. Bring mixture to a gentle simmer, then remove from heat. Stir in salt, pepper, and half the cheddar cheese until fully melted and smooth.
03 - Arrange half the sliced potatoes in the bottom of the prepared crockpot, overlapping slices slightly. Scatter half the chopped onion over the potatoes, followed by half of the remaining cheddar cheese. Pour half the cheese sauce evenly over the layer.
04 - Add the remaining potato slices, followed by the rest of the chopped onion and remaining cheese. Pour the remaining sauce over the top, ensuring coverage. Sprinkle with paprika if using.
05 - Cover and cook on high setting for 4 hours or low setting for 7 hours, until potatoes are fork-tender and the sauce is bubbly.
06 - Remove lid and let stand for 10 minutes to allow the sauce to thicken slightly. Garnish with chopped fresh parsley before serving.