Wash the apples, cut them in half, and scoop out the core with a spoon or melon baller to make a small well in the center.
In a small bowl, mix the melted butter, cinnamon, maple syrup, and oats or walnuts.
Fill each apple half with the cinnamon mixture, pressing it down gently so it stays put.
Place the apples in the air fryer basket cut-side up. Cook at 350°F (175°C) for 10 to 12 minutes, until the apples are soft and the topping is golden.
Serve warm. A scoop of vanilla Greek yogurt or a light drizzle of caramel takes it over the top.