• Preheat the oven to 400 degrees F.
• Spray an 8x8-inch baking dish with non-stick cooking spray.

For the Potatoes:

• On a cutting board, slice the potatoes into 1/4-inch slices and slice the onion into rings.
• Layer half of the onions into the bottom of the prepared dish.
• Add the potato slices.
• Top with the other half of the onions.
• Sprinkle the potatoes and onions with salt and pepper to taste.

For the Sauce:

• In a medium-size saucepan, melt the butter over medium heat.
• Whisk in the flour and salt, and stir constantly for one minute.
• Stir in the milk. Cook until mixture has thickened.
• Add in the cheese all at once, and continue stirring until melted, about 30 to 60 seconds.
• Pour the cheese sauce over the potatoes. Sprinkle on more salt and pepper if desired.
• Cover the dish with aluminum foil and place it on a baking sheet.
• Bake 90 minutes in the preheated oven.
• Remove the foil and broil for 2 – 5 minutes to brown the top, if desired.