Preheat the air fryer to 350°F (175°C) or the oven to 375°F (190°C).
In a bowl, toss the sliced apples with the melted butter, cinnamon and vanilla until evenly coated.
2 apples, 1 tbsp melted butter, 1 tsp cinnamon, ½ tsp vanilla extract
Dip the apples in the walnut flaxseed mixture.
¼ cup walnuts, 1 tbsp flaxseed
Spread the apples in an even layer in the air fryer basket or on a parchment-lined baking sheet.
Air fryer: Cook for 8–10 minutes, shaking the basket halfway through, until the apples are tender and the walnuts are lightly toasted.
Oven: Bake for 18–22 minutes, until the apples are soft and the walnuts are fragrant and golden.
Serve warm. If desired, add a dollop of Greek yogurt or cottage cheese for extra protein.