Slice the potatoes by hand, with a mandoline or using the slicing disk in a food processor. If you can’t get the slices to 1/8-inch thickness, it may take longer to cook.
1 t kosher or sea salt
1/2 t black pepper
2-1/2 T unsalted butter
3 lbs (peeled) baking potatoes (russets), cut into 1/8-inch thick slices
1 -2 T unsalted butter, melted, divided
1 T fresh parsley, chopped (optional)
1. Preheat oven to 450ºF. Combine salt and pepper in a small bowl.
2. Melt the 2-1/2 T unsalted butter in a 10-inch cast-iron or ovenproof heavy skillet over medium heat. Take off heat. Arrange a single layer of potato slices, slightly overlapping, in a circular pattern in pan; sprinkle with 1/4 t of the salt and pepper mixture. Drizzle a small amount (1/2 t or so) of melted butter over potatoes. Repeat layers 5 times (if you can), ending with butter. Press firmly to pack. Cover with foil or lid and bake for 20 minutes.
3, Uncover and bake an additional 25 minutes or until potatoes are golden. Loosen edges of potatoes with a spatula. Place a plate upside down on top of pan; invert potatoes onto plate. Sprinkle with parsley, if using.