Preheat the oven to 375º F.

Wash potatoes, leave skin on

Using a mandoline, thinly slice the potatoes into identical pieces.

Brush the bottom of a Staub/similar cast iron pan with some of the melted butter.

Arrange the potato slices vertically but at a slight angle in a circular pattern in the dish, filling it completely. Pour the remaining melted butter over the potatoes and sprinkle with the rosemary and salt.

Roast until the top edges of the potatoes are crispy, about 1 hr. If needed, broil for last 10 mins. Let stand for 10 mins before serving