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

Peel and slice the russet potatoes into ¼ inch thick rounds. Layer the potato slices in the prepared baking dish, alternating with thinly sliced onions.

Melt 4 tablespoons of butter in a medium saucepan over medium-low heat. Whisk in ¼ cup of flour and a pinch of salt until bubbly.

Gradually pour in 2 cups of warmed whole milk while stirring continuously. Cook and stir for about 3-5 minutes until the sauce thickens.

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

Pour the cheese sauce evenly over the layered potatoes and onions, then cover with aluminum foil and bake for 90 minutes.

For a golden top, remove the foil and broil for an additional 3-5 minutes, watching closely.

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