Best Scalloped Potatoes

Preheat oven to 400°F (200 degrees C).
Butter an 8×8 casserole dish.
Arrange the potatoes and onions in the prepared baking dish, staggering potato and then onion.
Season with salt and pepper to taste.
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 to 60 seconds.
Pour cheese over the potatoes, and cover the dish with aluminum foil.
Bake 1 1/2 hours (90min.) in the preheated oven.
For a crisp topping, change the oven setting to broil for 5 minutes before removing the dish.