01 - Generously butter the interior of the crockpot insert to prevent sticking.
02 - Melt 2 tablespoons 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 cream and milk while whisking constantly until smooth. Bring to a gentle simmer, then remove from heat. Stir in salt, pepper, and half the cheddar until completely melted.
03 - Arrange half the potato slices in the bottom of the prepared crockpot, overlapping slightly for even cooking. Distribute half the chopped onion evenly over potatoes, followed by half the remaining cheese. Pour half the sauce mixture over the layer.
04 - Repeat layering with remaining potatoes, onion, cheese, and sauce. Sprinkle paprika evenly over the top 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 bubbling.
06 - Remove from heat and let stand for 10 minutes to allow sauce to thicken slightly. Garnish with fresh parsley before serving.