Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish with unsalted butter.

In a medium saucepan, melt 4 tablespoons of unsalted butter over medium heat, then whisk in ¼ cup of all-purpose flour for about 1 minute to form a roux.

Slowly pour in 1 cup of heavy cream and 1 cup of milk, whisking until smooth. Season with garlic powder, salt, black pepper, and paprika; cook until thickened, about 3-5 minutes.

Layer half the sliced potatoes and onions in the baking dish. Pour half the cheese sauce over it, then sprinkle with half the cheddar cheese.

Repeat the layering process with the remaining potatoes, onions, cheese sauce, and cheddar cheese.

Cover the dish with aluminum foil and bake for 45 minutes. Remove the foil and continue baking for an additional 15-20 minutes until golden brown.

Allow the dish to cool for 10 minutes before serving to let layers set.