Preheat oven to 400º F. Spray a 1.5 quart casserole dish with cooking spray.

Layer 1/2 of the peeled and sliced potatoes into the bottom of the prepared casserole dish. Top potato layer with the onion slices, and then add the remaining potatoes onto the top. Season with salt and pepper to taste.

Melt butter over medium heat in a medium-sized saucepan. Mix in the flour and salt, and stir constantly with a whisk for one minute. Stir in milk. Whisk and cook until mixture has thickened.

Stir in cheese all at once, and continue stirring until melted, about 30 to 60 seconds. Taste and season accordingly.

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

Bake 1 1/2 hours in the preheated oven. When finished, potatoes should be bubbly and hot.

Remove foil and dish onto plates and enjoy!