01 - Generously grease the inside of the crockpot insert with butter to prevent sticking.
02 - Melt 2 tbsp butter in a small saucepan over medium heat. Add minced garlic and cook 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 to a simmer, then remove from heat. Stir in salt, pepper, and half the cheddar cheese until fully melted.
03 - Arrange half the sliced potatoes in the bottom of the crockpot, overlapping slices as needed. Spread half the chopped onion evenly over potatoes, followed by half of the remaining cheese. Pour half the sauce mixture over this 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.
06 - Let the scalloped potatoes rest for 10 minutes before serving to allow the sauce to thicken slightly. Garnish with fresh chopped parsley if desired.