• Preheat the oven to 400 degrees F.
• Spray two 20 ounce oven safe baking dishes (or an 8x8-inch square baking dish) with non-stick cooking spray.

For the Potatoes:

• Slice the potatoes into 1/4 inch slices and slice the onion into rings. A food processor works great for this.
• Layer half of the onions into the bottom of the prepared dishes, divided evenly.
• Add the potato slices, divided evenly between the dishes.
• Sprinkle the potatoes with salt and pepper to taste.
• Top with the other half of the onions, divided evenly.

For the Sauce:

• 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.
• Add in the cheese all at once, and continue stirring until melted, about 30 to 60 seconds.
• Pour the cheese sauce over the potatoes, divided evenly. Sprinkle with more salt and pepper if desired.
• Cover the dishes with aluminum foil.
• Place the two dishes on a baking sheet.
• Bake 1 1/2 hours in the preheated oven.
• Broil for 2 – 5 minutes to brown the top, if desired.