Preheat oven to 350 degree F and grease a 13"x9" baking pan with cooking spray.
Bring a large pot of water to a boil. Scrub potatoes and place them, whole, into the boiling water for 20-25 minutes. They don't need to be fully cooked, but should be easily pierced with a sharp knife. Remove from the water and let sit until cool enough to handle.
Meanwhile, make the cheese sauce. Melt butter in a medium saucepan over medium heat. Whisk in flour and cook, whisking constantly, until flour is golden brown; about 2 minutes. Stir in milk and cook, stirring often, until thickened; about 2-3 minutes. Remove from heat and stir in shredded cheese. Season with salt and pepper.
Gently peel potatoes and slice into 1/8 inch rounds.
Place 1/3 of the potatoes overlapping in a single layer in the baking dish, seasoning with salt and pepper. Spoon about 1/3 of the cheese sauce on top of the potatoes. Repeat for two more layers. Pour all of the remaining cheese sauce over the top layer of potatoes. Spread to ensure all of the potatoes are covered.
Sprinkle with 1/2 cup shredded cheddar (if desired) and paprika (about 1/8 teaspoon, just for color).
Bake in the 350 degree oven for 20-25 minutes, or until sauce is bubbly.
Garnish with fresh chopped chives or parsley, if desired. Serve immediately.
If the cheese sauce seems too thick, add more milk, just a little bit at a time, whisking it each time until you like the consistency. It should be thick, but still somewhat pourable. Potatoes should be able to be pierced through with a sharp knife, but not falling apart.