01 - Generously butter the interior of the slow cooker insert to prevent sticking.
02 - Melt 2 tbsp butter in a medium saucepan over medium heat. Add minced garlic and sauté until fragrant, about 1 minute. Whisk in flour and cook for 1 additional minute. Gradually pour in cream and milk while whisking constantly until smooth. Bring mixture to a simmer, then remove from heat. Stir in salt, pepper, and half the cheddar cheese until fully melted and incorporated.
03 - Arrange half the sliced potatoes in an even layer in the prepared crockpot, overlapping slices slightly as needed. Distribute half the chopped onion evenly over potatoes, then top with half of the remaining cheddar cheese. Pour half the cheese sauce 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 completely tender when pierced with a fork.
06 - Remove lid and let the scalloped potatoes rest for 10 minutes to allow sauce to thicken slightly. Garnish with chopped fresh parsley before serving.