Preheat oven to 400°F. Lightly spray an 8x8-inch casserole dish with nonstick cooking spray.

Arrange the potatoes and onions in the prepared baking dish, staggering potato and then onion.

In a medium-size saucepan, melt butter over medium heat. Mix in the flour and salt, and stir constantly with a whisk for one minute. Stir in milk. Cook until mixture has thickened.

Stir in cheese all at once, and continue stirring until melted and smooth, about 30-60 seconds.

Pour the cheese sauce over the potatoes, and cover the dish with aluminum foil.

Bake for 85-90 minutes, or until the potatoes are tender.

For a crisp topping, change the oven setting to broil after the potatoes are baked. Broil until golden brown.

Season with salt and pepper to taste and serve.