For the apple: Peel and core the apple. Chop into small pieces. Add apple pieces to a large microwave-safe mug and stir in brown sugar and cinnamon.
For the crisp: In a separate small bowl, stir together butter, brown sugar, flour and oats until well combined.
Sprinkle the crisp mixture over the apples and gently press down. Microwave for 2 minutes (check after 90 seconds) or until the apples are tender. Serve with whipped cream or ice cream, if desired.
