Preheat oven to 400 degrees F.
Wash potatoes and slice into 1/8' thin slices. Slice onion as thin as possible.
Layer potatoes and onions in little stacks. Place stacks upright in a greased casserole dish.
Melt butter & flour in a sauce pan and cook 2-3 minutes. Add seasonings and milk. Whisk over medium heat until thick and bubbly. Remove from heat and stir in cheeses until melted.
Spoon cheese sauce over potatoes. Cover with foil (sprayed with cooking spray) and bake 60 minutes. Remove foil and bake an additional 20-30 minutes or until lightly browned and potatoes are cooked.