Preheat the air fryer to 350°F (175°C).
In a small bowl, toss the diced apples with granulated sugar, 1/4 teaspoon of cinnamon, and lemon juice.
Transfer the apple mixture into a small oven-safe ramekin or mini pie dish.
In the same bowl, combine the oats, flour, brown sugar, remaining 1/4 teaspoon cinnamon, and salt.
Cut the cold butter into the oat mixture using a fork or fingers until it forms coarse crumbs.
Sprinkle the crumble mixture evenly over the top of the apples.
Place the ramekin in the air fryer basket and cook for 12 to 15 minutes, or until the apples are tender and the topping is golden brown.
Carefully remove the ramekin and let cool for 5 minutes before serving.