Start by peeling and slicing 1-2 firm apples into ⅛ inch thick pieces. In a small bowl, toss the apple slices with melted butter, brown sugar, cinnamon, and flour until evenly coated.

In another bowl, combine softened butter, flour, oats, brown sugar, and a sprinkle of cinnamon. Mash the softened butter into the dry ingredients until crumbly.

Layer half of the apple mixture at the bottom of a microwave-safe mug, sprinkle half of the topping over the apples, and repeat with the remaining apple mixture and topping.

Cook in the microwave on high for about 3 minutes, keeping an eye on it until the filling bubbles and the topping starts to brown.

Carefully remove the mug from the microwave, let it cool for a couple of minutes, and serve warm, ideally with vanilla ice cream or whipped cream.