Preheat your oven to 375°F (190°C).
Thinly slice the russet potatoes using a mandolin or sharp knife.
In a saucepan, melt the butter over medium heat and add the flour, stirring continuously.
Gradually whisk in the heavy cream and whole milk until smooth.
Stir in garlic powder, onion powder, and season with salt and pepper.
Remove the saucepan from heat and stir in the shredded cheddar cheese until melted.
In a greased baking dish, layer half of the sliced potatoes.
Pour half of the cheese sauce over the first layer of potatoes.
Repeat with the remaining potatoes and cheese sauce.
Sprinkle grated Parmesan cheese on top.
Cover the baking dish with aluminum foil and bake for 45 minutes.
Remove the foil and bake for an additional 20-30 minutes, or until golden brown.
Let the dish cool slightly before serving.