Prepare the Potatoes

1. Preheat Oven: Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter or cooking spray.
2. Slice the Potatoes: Peel and thinly slice the russet potatoes (about 1/8-inch thick). If using, slice the onion as well.
3. Layer the Potatoes: Place a layer of sliced potatoes in the prepared baking dish, followed by a layer of onions (if using). Repeat layers until all potatoes and onions are used.

Make the Cheese Sauce

1. Melt Butter: In a medium saucepan over medium heat, melt the butter.
2. Add Flour: Whisk in the flour and cook for 1-2 minutes, stirring constantly to form a roux.
3. Add Milk: Gradually whisk in the milk, continuing to stir until the mixture thickens (about 5-7 minutes).
4. Add Cheese and Seasoning: Remove from heat, then stir in the shredded cheese, garlic powder, salt, pepper, and paprika until melted and smooth.

Assemble and Bake

1. Pour Sauce Over Potatoes: Pour the cheese sauce evenly over the layered potatoes in the baking dish, ensuring all slices are covered.
2. Bake: Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes. Then, remove the foil and bake for an additional 10-15 minutes, or until the potatoes are tender and the top is golden brown.
3. Cool and Serve: Let cool for a few minutes before serving. Enjoy your cheesy goodness!