Preheat the oven to 375°F (190°C). Grease a baking dish with butter.

Peel and thinly slice the Russet potatoes using a knife or mandoline.

In a saucepan, melt the butter over medium heat. Stir in the flour and cook for 1-2 minutes until lightly golden.

Gradually whisk in the heavy cream and milk, stirring constantly until the mixture thickens.

Add garlic powder, onion powder, salt, and black pepper. Remove from heat and stir in half of the cheddar and Parmesan cheese until melted.

Layer half of the sliced potatoes in the greased baking dish. Pour half of the cream sauce over the potatoes.

Repeat with the remaining potatoes and cream sauce. Top with the remaining cheddar and Parmesan cheese.

Cover with foil and bake for 45 minutes. Remove the foil and bake for an additional 15 minutes until golden and bubbly.

Let cool for a few minutes before serving.