Preheat the oven to 350 degrees.
In a small sauce pan, melt butter. Once melted add flour with a fork – whisking together.
Next, whisk in milk.
Cook sauce on low until smooth and boiling, whisking until thickened.
Add in salt, pepper , and dry mustard and stir well.
Reduce heat to low and stir in 1 cup of the cheese.
Place a half of the sliced potatoes in a lightly greased 1.5 quart casserole dish (8×8 or 9×9 will work)
Pour half of the cheese sauce over potatoes.
Add a second layer of potatoes and the rest of the cheese sauce to the top of that layer
Sprinkle the remaining 1/2 cup of cheese on top.
Sprinkle the top with paprika.
Bake at 350 degrees, uncovered for 1 hour.