Slice Potatoes into 1/4 inch thick disks. A mandolin works great for this!
Mill the flour so it is ready to go.
Also, make sure your cheese is shredded. (I highly recommend shredding the cheese yourself, and not using pre-shredded cheese for texture and flavor.)
Preheat oven to 400*F.
I used an 8x8 glass pan, but you could also make this in a cast iron skillet, or double the recipe for a 9x13 pan. But, make sure to generously butter any pan you use to prevent sticking.
Place sliced potatoes into the prepared pan and make them even. Don't worry bout how they are stacked, the cheese sauce does a great job at working it's way all throughout the dish.
In a medium sauce pan, make the Roux. To Make the Roux melt butter, then mix in the freshly milled flour, salt, and stir consistently with a whisk for about one minute. Then, stir in the milk, and cook for about 4-6 more minutes, until the mixture has thickened a bit.
After the roux has thickened, then pour in the shredded cheese all at once. Stir until it is all melted and smooth. This may take a minute or so.
Then, pour the cheese sauce over the sliced potatoes.
Cover the dish with foil
Bake covered for 85-90 minutes. The potatoes should be nice and tender.
To brown the top, remove the foil, and then turn up the oven to broil. Watch closely, this can happen fast in some ovens. Broil until the potatoes are browned to your liking.
Allow to cool for about 5-10 minutes in the pan before serving.
Serve, and season with salt and pepper to taste.