01 - Butter the interior of the crockpot insert thoroughly 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. Slowly whisk in cream and milk until fully incorporated and smooth. Simmer briefly until thickened, then remove from heat. Stir in salt, pepper, and half the cheddar until melted and creamy.
03 - Arrange half the potato slices in the bottom of the prepared crockpot, overlapping slightly for even cooking. Scatter half the chopped onion over the potatoes, followed by half the remaining cheddar. Drizzle half the cheese sauce evenly over the layer.
04 - Repeat the 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 for 7 hours, until potatoes are completely tender when pierced with a fork.
06 - Remove the lid and let the scalloped potatoes rest for 10 minutes to set. Sprinkle with fresh parsley before serving.