Heat oven to 400° F. Line a square 8-inch rimmed baking sheet with parchment paper.
Cut the apple into 12 ¼-inch slices, discarding the core.
Place the apple slices in the prepared pan in a single layer. Drizzle them with the melted butter and use your hands to coat them. Sprinkle the tops of the slices with half the cinnamon.
Bake for 10 minutes.
Turn the apple pieces to the other side. Sprinkle them with the remaining cinnamon. Keep baking them until golden-brown and tender, about 10 more minutes.