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 minute to form a roux. 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 melted and well combined.
03 - Arrange half the sliced potatoes in the bottom of the prepared crockpot, overlapping slices slightly. Spread half the chopped onion over the potatoes, followed by half of the remaining cheese. Pour half the cheese sauce evenly 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 - Let stand uncovered for 10 minutes to allow the sauce to thicken slightly. Garnish with fresh chopped parsley before serving.