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

Peel russet potatoes and slice them into ¼-inch rounds. Slice yellow onions if using, and layer them among the potatoes in the dish.

Melt 4 tablespoons of butter in a saucepan over medium-low heat. Whisk in ¼ cup of flour and a sprinkle of salt, cooking until it forms a roux.

Gradually pour in 2 cups of warmed whole milk, stirring constantly until the mixture thickens and bubbles gently.

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 each layer is evenly coated.

Cover with aluminum foil and bake for 90 minutes, checking tenderness halfway through.

Remove foil and broil for 3 to 5 minutes for a golden top, monitoring closely to prevent burning.