Peel and thinly slice the potatoes, place in a bowl of cold water and let sit while making the white sauce.
In a medium/large pot on low/medium heat melt the butter, add the flour and salt and whisk until smooth. Slowly add the milk, stirring constantly until thickens, approximately 2-3 minutes. Remove from heat.
Drain and towel dry the potato slices.
Place 2-3 tablespoon of white sauce on the bottom of baking pan**, add 1/3-1/2 of the potatoes and 1/3 of the white sauce continue for another 1-2 levels.
Sprinkle with Parmesan cheese if desired, and bake covered for 45 minutes, then uncover and bake for an additional 30 minutes. Let sit 5 minutes before serving. Sprinkle with chopped parsley if desired. Enjoy!
**I used a medium baking pan (10x7 inches / 26x17 centimeters) and made 3 levels, you could use a larger pan and make only 2 layers if desired.