Preheat the oven to 350°F (175°C).

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

In a saucepan, melt the butter over medium heat. Add the chopped onion and sauté until translucent.

Stir in the flour and cook for 1-2 minutes.

Gradually whisk in the milk and cream, cooking until the mixture thickens.

Add 1 1/2 cups of cheddar cheese, salt, pepper, and garlic powder. Stir until the cheese is melted and smooth.

In a greased baking dish, layer half of the sliced potatoes. Pour half of the cheese sauce over the potatoes.

Repeat with the remaining potatoes and sauce.

Sprinkle the remaining cheddar cheese on top.

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

Let it cool for a few minutes before serving.