Prep your air fryer: Preheat to 350°F (175°C) for 3–5 minutes. This helps the topping crisp evenly.
Make the apple filling: In a large bowl, toss sliced apples with sugars, lemon juice, cinnamon, nutmeg, vanilla, cornstarch, and a pinch of salt until coated and glossy.
Mix the topping: In a separate bowl, combine oats, flour, brown sugar, cinnamon, and salt.
Add cold butter and rub it in with your fingertips (or use a pastry cutter) until you get pea-sized crumbs. Stir in nuts if using.
Assemble: Lightly butter a 6–7 inch oven-safe dish or 4 small ramekins that fit your air fryer basket. Spoon in the apple mixture and spread into an even layer.
Sprinkle the crumble over the top, leaving a few small gaps for steam to escape.
Air fry: Place the dish in the air fryer basket. Cook at 350°F (175°C) for 16–22 minutes, until the topping is golden brown and the filling is bubbling at the edges. Start checking at 15 minutes; ramekins may finish sooner.
Rest briefly: Let the crisp sit for 5–10 minutes.
This allows the juices to thicken and the topping to set.
Serve: Scoop into bowls. Add ice cream, whipped cream, or a warm drizzle of caramel for extra comfort.