Preheat the oven to 375 degrees F.

Brush 2 large baking sheets lightly with oil. Use a mandoline or hand held slicing machine to cut the potatoes lengthwise into 1/8-inch thick slices. Arrange the slices in 1 flat layer on the baking sheets. Brush the slices lightly with oil and bake until golden throughout, 15 to 20 minutes, checking often since they brown at different rates. Transfer to paper towels and sprinkle with salt and the rosemary while hot.