Preheat the oven to 400°F.

Lightly grease a 2-quart casserole dish with cooking spray (this is slightly smaller than a standard 9×13-inch pan).

Peel and thinly slice the potatoes. Arrange them evenly in the prepared dish.

Slice the onion and tuck 1–2 slices between each layer of potatoes. Season the potatoes and onions generously with salt and pepper. Set aside.

In a small saucepan over medium heat, melt the butter. Whisk in the flour and cook for about 1 minute, until smooth.

Slowly whisk in the milk, a little at a time, until fully combined.

Reduce heat to medium-low and continue whisking until the sauce thickens.

Lower the heat and stir in the shredded cheese until melted and smooth.

Pour the cheese sauce evenly over the potatoes. Cover tightly with foil.

Bake for 1 hour and 15 minutes. Remove foil and bake an additional 15 minutes, until bubbly and lightly golden.

Let cool for 5–10 minutes before serving. Top with sliced scallions if desired.