Peel the potatoes and cut in half lengthways. Place the halves on the cut surface and cut 2/3 fan-like. Bake the potatoes in a preheated oven at 160 degrees.
The potatoes should now have fanned out a bit after they have been cooked. Now pour the grated parmesan and a little rosemary on top and bake for another 10-15 minutes.