Scrub potatoes and cut out any impurities. Slice potatoes thinly, mandolins are handy for this. Make sure they are ¼ thickness or thinner.
In a sauce pan or skillet, melt butter and whisk in flour. Let bubble for 1-2 minutes on med heat.
Whisk in milk.
When sauce is smooth, add in ½ cup sharp cheese, all of the Gruyere cheese, salts, parsley, bacon, chives (if you want to top potatoes with bacon and chives reserve little bit for topping).
Grease a 9x13 baking dish. Place potatoes in. Pour sauce over. Slighty shift the potatoes to help cream evenly distribute and coat the potaotes. You want sauce in between each potato.
Top with ½ cup remaining cheese plus any bacon or chives you reserved.
Bake covered with foil at 400 degrees F for 25 mins. Uncover and bake for an addition 15-20 minutes until potatoes are cooked thru and cheese is bubbly.
Serve warm.
