Combine the brown sugar, flour and cinnamon in a small bowl. Thinly slice the apple and toss in sugar mixture to coat.
In a separate small bowl, combine the oats and butter. (I prefer old fashioned oats, even though the original recipe called for instant. I add enough oats to absorb all the butter, which was more than the 1 Tbls. called for in the original recipe.).
Place the apples in a microwave safe mug and top with the oat mixture. Microwave 2-3 minutes or until the apples are tender.
Top with a small scoop of ice cream, or whipped cream and a drizzle of caramel sauce, or sprinkle with shredded cheddar cheese before serving, if desired.