Preheat the air fryer to 350 degrees for at least 5 minutes and grease 4 (6 ounce) ramekins with butter or nonstick cooking spray.
While preheating, mix the brown sugar, flour, oats, walnuts, cinnamon, and nutmeg together in a medium sized mixing bowl.
Melt the butter for 30 seconds in the microwave and add to the mixture. Mix together to make a crumble mixture.
In a medium mixing bowl place the diced apples and mix in half of the crumble mixture.
Divide the apple mixture equally among the 4 ramekins.
Break up the remaining crumble mixture and sprinkle it equally on top of the apple mixture in the ramekins. Cover each ramekin tightly with aluminum foil.
Set the ramekins in the air fryer and cook at 350 degrees for 20 minutes.
Carefully remove all of the foil, then replace the ramekins in the air fryer and cook for another 5 minutes or until the topping is golden brown and bubbly.
Remove the ramekins and serve immediately, topping each ramekin with a scoop of ice cream, optional.