Place sliced potatoes and onions into casserole dish alternating between potato and onion
Melt butter in sauce pan over medium heat. Mix flower and salt and continuously stir with a whisk for one minute. Stir in like cooking until thickened.
Stir in cheese all at once and cautiously stir until melted 30 to 60 seconds
Pour cheese mixture over potatoes and cover dish with aluminum foil.