Wash the potatoes. Leave the skin on. Cut the sweet potatoes into thin slices with a cucumber slicer. Alternatively, it works's also with a sharp knife.
Put the potato slices in a bowl. And mix them with the olive oil. This works best with your hands.
Chop the rosemary needles very small.
Put the potato pieces on the baking tray. Sprinkle them with salt, pepper and rosemary.