Wash and optionally peel 3 large russet potatoes. Using a sharp knife or mandoline, slice each potato crosswise into ⅛-inch thick slices, keeping slices stacked in order. Transfer slices to a large bowl, cover completely with cold water, and soak for 30 minutes to remove excess starch. Drain and rinse briefly.
Pat potato slices completely dry with paper towels, pressing firmly to absorb all surface moisture. Let air-dry for 5 minutes if possible. Transfer dried slices to a clean bowl, add olive oil and kosher salt, and toss thoroughly to coat every surface.
Thread all slices from one potato onto two parallel metal skewers spaced about 1 inch apart. Gently fan the slices open, creating visible space between each slice for air circulation. Repeat with remaining potatoes.
Preheat oven to 425°F. Line a rimmed baking sheet with foil and place a wire rack on top. Position the three skewered accordion potatoes on the rack with space between them. Bake for 25 minutes.
Carefully flip each potato 180 degrees using tongs. Return to oven and bake 20 to 25 minutes more until deep golden brown and crispy on all surfaces, with total baking time of 45 to 50 minutes.
Remove from oven and brush with melted butter if using. Sprinkle with black pepper, garlic powder, paprika, and Parmesan cheese. Switch oven to broil on high and position rack 6 inches from heating element.
Broil for 2 to 4 minutes, watching constantly, until edges are deeply golden with blistered spots and cheese is melted and bubbling. Remove immediately when properly browned.
Let rest on rack for 3 to 5 minutes to set the crispy texture. Slide potatoes off skewers onto a serving platter. Garnish with flaky salt and fresh herbs if desired. Serve immediately while hot and crispy.