Heat your oven to 400 degrees F. Line a square 8-inch rimmed baking sheet with parchment paper.
Cut the apple into 12 ¼-inch slices, discarding the core, as shown in the video below.
Place the apple slices in the prepared pan. Drizzle them with the melted butter and use your hands to coat them. Arrange them in the pan in a single layer.
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. Drizzle with the pan juices and serve.
There's no need to peel the apple - the skin softens nicely in the oven. However, if you wish, you can certainly peel it. You can skip the butter if you wish. The butter does enhance the apple's flavor, but I find that the apple slices are delicious even without it. If you do use butter, make sure it's unsalted.