Preheat oven to 350°F and grease a 13x9" baking pan with cooking-spray.
Bring a large-pot of water to a boil. Scrub potatoes and set them whole, in the boiling water. Cook for approximately 20minutes. Remove from the water & let sit until cool enough to handle.
Make the cheese-sauce. Melt butter in a medium saucepan over moderate heat. Whisk in lour & cook, whisking constantly, until flour is golden-brown, roughly 2minutes. Stir in milk and cook, stirring frequently, until thickened roughly 2-3minutes. Remove from heat & stir in shredded cheese. Season with pepper and salt.
Gently peel potatoes & slice to ⅙" rounds. Place ⅓ of this potatoes overlapping in one layer in the baking dish, seasoning with pepper and salt. Spoon about ⅓ of this cheese-sauce in addition to the potatoes.
Repeat for 2 more layers. Pour each the rest of the cheese-sauce on the top layer of potatoes. Spread to guarantee each the potatoes are covered.
Sprinkle with ½ cup shredded-cheddar (if desirable) and paprika (about ⅛ tsp only for colour).
Bake in the 350° oven for 20-25minutes, or until sauce is bubbly.
Garnish with fresh-chopped chives or parsley, if desired. Serve immediately.