01 - Generously butter the interior of the crockpot insert to prevent sticking and ensure easy serving.
02 - Melt 2 tablespoons butter in a medium saucepan over medium heat. Add minced garlic and sauté until fragrant, approximately 1 minute. Sprinkle in flour, stirring constantly for 1 minute to cook out the raw taste. Gradually whisk in heavy cream and whole milk until completely smooth. Bring the mixture to a gentle simmer, then remove from heat. Stir in salt, black pepper, and half the shredded cheddar cheese until fully melted and incorporated. Set sauce aside.
03 - Arrange half the sliced potatoes in the bottom of the prepared crockpot, overlapping slices slightly for even cooking. Distribute half the chopped onion evenly over the potatoes, followed by half of the remaining cheddar cheese. Pour half the prepared cheese sauce over this layer.
04 - Repeat the layering process with remaining potatoes, onion, cheese, and sauce. Sprinkle paprika across the top surface if using, for added color and flavor.
05 - Cover and cook on high setting for 4 hours or low setting for 7 hours, until potatoes are completely tender and easily pierced with a fork. The edges should be golden and bubbly.
06 - Remove lid and allow the dish to rest for 10 minutes before serving. This helps the sauce thicken slightly and makes serving easier. Garnish with fresh chopped parsley if desired.