6 tablespoons unsalted butter, melted, divided
3 1/2 pounds Idaho potatoes (4-6 large)
24 (about) fresh or dried bay leaves
Kosher salt and fleur de sel
Preheat oven to 425°F. Brush a 13x9x2" baking dish or cast-iron griddle with 2 tablespoons butter. Peel potatoes and trim ends (do not rinse). Trim all 4 sides of potatoes to form a rectangle. Using a mandoline, cut potatoes crosswise into 1/8" slices, keeping slices in stacks as best you can.
Re-form slices from each potato into a stack. Place in prepared dish, fanning apart slightly like a deck of cards. Insert bay leaves between potato slices at even intervals. Season with salt and drizzle with remaining 4 tablespoons butter.
Bake potatoes, rotating the dish halfway through cooking, until the edges are crisp and golden and the centers are tender, about 1 hour. Sprinkle with fleur de sel.