Scrub potatoes. With a sharp knife, slice potatoes thinly but not all the way through, leaving slices attached at the bottom. Place on rimmed baking sheet.
Drizzle with melted butter. Combine the parmesan cheese, rosemary, salt and pepper; sprinkle over potatoes and between slices.
Bake for 45 minutes, basting with butter from bottom of pan every 15 minutes.
Top with cheddar cheese and bake for 5-10 minutes more.