Preheat air fryer to 375 degrees for 3 to 5 minutes. Grease a ramekin with oil or butter.
Prepare apple by washing it, removing the core, and cutting into half inch cubes. Leave the skin on or off depending on your preference.
In a small bowl. melt butter in the microwave. Then, add brown sugar, cinnamon and sliced apples. Gently toss until apples are well-coated in cinnamon-sugar mixture. Layer apple pieces in the bottom of the ramekin.
In a small bowl, add flour, oats, brown sugar and cinnamon. Mix well. Cut the cold butter into small pieces and then press it into the dry mixture using your fingers or a fork. Continue until the butter is well incorporated and a crumbly topping forms. Sprinkle the topping over the apples evenly.
Place ramekin in preheated air fryer and cook for 15-20 minutes or until the apples are fork-tender and the topping is golden brown.
Serve hot with a scoop of ice cream and/or whipped cream.
