Lightly grease two 6-ounce ramekins with butter or cooking spray.
In a small bowl, toss the diced apples with the brown sugar, cinnamon, nutmeg, lemon juice, and vanilla until evenly coated. Let sit 2 to 3 minutes.
Divide the apple mixture between the ramekins, filling each about three-quarters full and pressing down gently.
In a separate bowl, combine the oats, flour, brown sugar, and salt. Add the cold cubed butter and work it in with your fingertips or a fork until the mixture resembles coarse sand with pea-sized clumps.
Spoon the topping over the apples, mounding slightly. Cover each ramekin tightly with foil.
Place the ramekins in the air fryer basket. Cook at 350°F for 10 minutes.
Remove the foil and cook for 5 more minutes, until the topping is deep golden and the filling bubbles at the edges.
Let rest 5 minutes before serving. Top with vanilla ice cream or whipped cream.