Preheat oven to 375°F and lightly grease a baking dish.
Melt butter in a saucepan, whisk in flour, and cook for one minute.
Gradually add milk and cream while whisking until smooth and thickened.
Stir in 1 ½ cups cheddar cheese, garlic powder, salt, and pepper until melted.
Layer half of the sliced potatoes in the dish, pour half of the sauce over them.
Repeat with remaining potatoes and sauce.
Sprinkle the top with the remaining cheese.
Cover with foil and bake for 30 minutes, then uncover and bake another 25 minutes until golden and bubbly.
Let rest 10 minutes before serving, garnish with parsley.