Prep the air fryer. Preheat your air fryer to 350°F (175°C) for about 3 minutes. Grease a small, oven-safe baking dish or 4 small ramekins that fit in your air fryer basket.

Slice the apples. Peel (if you like) and thinly slice the apples, about 1/4-inch thick. Thin slices cook faster and more evenly.

Make the filling. In a bowl, toss apples with lemon juice, granulated sugar, 1/2 teaspoon cinnamon, vanilla, cornstarch, and a pinch of salt. Mix until the apples are evenly coated and glossy.

Arrange the apples. Spread the apples in an even layer in your prepared dish or divide among ramekins. Press down lightly to remove large air gaps.

Prepare the crumble topping. In a separate bowl, combine flour, oats, brown sugar, remaining cinnamon, nutmeg (if using), and a pinch of salt. Add the cold butter cubes. Use your fingers or a pastry cutter to rub the butter into the dry ingredients until the mixture looks like coarse crumbs with some pea-sized bits.

Top the apples. Sprinkle the crumble evenly over the apples, covering them completely. Don’t pack it down too much; a loose topping crisps better.

Air fry. Place the dish in the air fryer basket. Cook at 350°F (175°C) for 14–18 minutes. Start checking at 12 minutes. The topping should be deep golden and crisp, and the apples should be bubbling around the edges.

Test for doneness. Pierce the apples with a fork. They should be tender but not mushy. If needed, cook 2–4 minutes more, tenting with foil if the top browns too fast.

Rest before serving. Let the crumble sit for 5–10 minutes. This helps the juices thicken so each scoop holds together.

Serve. Spoon into bowls and add vanilla ice cream, whipped cream, or a drizzle of caramel. Enjoy warm.