01 - Generously grease the inside of the crockpot insert with butter to prevent sticking.
02 - In a small saucepan over medium heat, melt 2 tbsp butter. Add garlic and cook until fragrant, about 1 minute. Stir in flour and cook for 1 more minute to form a roux. Gradually whisk in cream and milk until smooth. Bring to a simmer, then remove from heat. Stir in salt, pepper, and half the cheddar cheese until melted; set aside.
03 - Layer half the sliced potatoes in the crockpot, overlapping as needed. Top with half the chopped onion and half of the remaining cheese. Pour half the sauce evenly over this layer.
04 - Repeat with remaining potatoes, onion, cheese, and sauce. Sprinkle paprika on top if desired for a subtle smoky flavor and color.
05 - Cover and cook on high for 4 hours or low for 7 hours, until potatoes are completely tender when pierced with a fork.
06 - Let the dish rest uncovered for 10 minutes before serving to allow the sauce to thicken slightly. Garnish with fresh parsley if desired.