Preheat your oven to 400°F (200°C) and coat an 8-inch square baking dish with non-stick spray.

Peel and slice the russet potatoes into ¼ inch rounds. Thinly slice the yellow onion if using, and layer them in the baking dish.

Melt the butter in a medium saucepan over medium-low heat. Whisk in the flour and salt for about 1 minute to form a roux.

Gradually pour warmed milk into the roux while stirring continuously. Cook for about 3-5 minutes until the sauce thickens.

Remove the saucepan from heat and stir in the shredded cheddar until fully melted.

Pour the cheese sauce over the layered potatoes and onions. Ensure all layers are coated.

Cover with foil and bake for 90 minutes until the potatoes are tender.

Remove foil and broil for 3-5 minutes until golden and bubbly.

Let the dish rest for 10-15 minutes before serving.