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

Peel and slice russet potatoes into ¼ inch thick rounds. Optionally, slice the yellow onion thinly. Layer potato slices and onion in the bottom of the baking dish.

In a medium saucepan, melt 4 tablespoons of butter over medium heat. Whisk in ¼ cup of all-purpose flour and a pinch of salt, stirring constantly for about 1 minute to create a roux.

Gradually pour in 2 cups of warmed whole milk, stirring continuously for about 3 to 5 minutes until the mixture thickens.

Remove from heat and stir in 2 cups of shredded cheddar cheese until melted and smooth.

Pour the cheese sauce over the layered potatoes and onions, ensuring thorough coverage.

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

Remove the foil and broil for an additional 3-5 minutes until the top is bubbly and golden brown.