Preheat oven to 375 degrees. Spray 9x13” casserole dish with nonstick cooking spray and set aside. In a medium saucepan over medium heat, add butter and cook unctions until tender. About 5 minutes.
Stir in flour, salt, and pepper until blended. Gradually stir in the milk and cook, stirring constantly, until mixture thickens.
Optional: If you wish to add cheese to make it more like Potatoes Au Gratin then add in this step:When the sauce just begins to thicken slightly, reduce heat, stir in the shredded cheese at this point.
In a buttered casserole or Pyrex dish, arrange half of the potatoes in a layer evenly, pour half of the sauce on top, repeat.
Sprinkle the top with paprika. Cover with foil and bake 45 minutes, then uncover and bake another 15 minutes or until potatoes are tender and it’s starting to brown on top.
Remove from the oven and allow to rest/cool for 15 minutes to allow the sauce to thicken making serving easier. Serve warm.