Preheat the oven to 400°F. Line a square 8-inch rimmed baking dish with high-heat-resistant parchment paper.
Cut the apple into twelve ¼-inch slices, discarding the core.
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 the apples for 10 minutes.
Flip the apple pieces and sprinkle them with the remaining cinnamon.
Keep baking them until golden brown and tender, for about 10 more minutes. Drizzle with the pan juices and serve.
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Notes
There's no need to peel the apple. The skin softens nicely in the oven. However, if you wish, you can peel it.
You can skip the butter if you wish. The butter does enhance the apple's flavor, but the apple slices are delicious even without it. If you use butter, make sure it's unsalted.
You can keep the leftovers in a sealed container in the fridge for 3-4 days. Serve them cold or reheat them in the microwave, covered, at 50% power.