Preheat oven to 400?F. Lightly spray an 8?8-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-sized saucepan, melt butter over medium heat. Mix in flour and salt, stirring constantly with a whisk for one minute.

Stir in milk and cook until the mixture has thickened.

Add 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.