01 - Generously grease the interior of the crockpot insert with butter to prevent sticking
02 - Melt 2 tbsp butter in a medium saucepan over medium heat. Add minced garlic and sauté until fragrant, approximately 1 minute. Whisk in flour and cook for 1 minute to form a roux. Gradually pour in heavy cream and whole milk while whisking constantly until smooth. Bring mixture to a simmer, then remove from heat. Stir in salt, pepper, and half the shredded cheddar until completely melted and incorporated
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 prepared cheese sauce over the layer
04 - Repeat layering with remaining potatoes, onion, cheese, and sauce. Dust the top 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 sauce is bubbly
06 - Remove lid and allow dish to rest for 10 minutes to set. Garnish with fresh parsley just before serving