Preheat the oven to 400°F. Line an 8-inch square 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 a single layer.
Sprinkle the tops of the apple slices with half the cinnamon. Bake 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.
You can freeze these apples, but the texture will change once thawed. Apples contain a lot of water, and freezing creates ice crystals that soften the flesh. Since they’re already tender from baking, expect them to become softer and perhaps a bit mushy. That said, they still freeze well and taste great. Just let them cool completely, freeze them in a single layer on wax paper, and then - once frozen - transfer to a freezer-safe container. They’re best used in cooked dishes or as a topping for yogurt rather than eaten as-is.