Make the Topping: In a small bowl stir together the butter, oats, brown sugar, flour, and cinnamon. Set aside.
Peel and chop the apple into bite-size chunks. Place 1/3 of the topping in the bottom of a microwave-safe mug. Next, fill the mug with the chopped apple. Finally, spoon the remaining oat topping over the apples. Your mug will be very full but it will cook down in the microwave
Microwave for 1 minute. With the tines of a fork, check the apples' softness. Continue cooking in 15-second increments until the apples are at the desired softness.
Remove the mug from the microwave carefully as it will be hot. Let it cool slightly.
Notes
Bulk Topping Idea:
Make a bulk topping and keep in the freezer for last-minute and quick apple crisps in a mug. The recipe is in the post above.
Serving Ideas:
Vanilla Ice Cream
Whipped Topping
Cooking Times
Cooking times will vary depending on the power of your microwave and the depth of your mug. I tend to use deep mugs and my microwave is not super powerful.