Peel, core, and slice apple into wedges. Slice each wedge into 1/8-inch thick pieces. Place apple into a bowl. Add sugar, cinnamon, and flour. Combine.
Place the butter in a small microwaveable plate. Microwave for 20 seconds to melt.
Meanwhile, spray a 1-cup capacity microwavable ramekin or mug with cooking spray. Add apple. Mound apple as high as you can without overflowing. They will cook down. There may be some excess depending on how large the apple was.
Remove the plate of butter from the microwave. Place the apple in the microwave. Cook for 2 1/2 minutes.
While the apple cooks, add the remaining crumb topping ingredients to the melted butter. Mix to form wet coarse crumbs. Spread crumbs into an even layer.
After the apple has cooked for 2 1/2 minutes, place the crumb topping plate beside the apple filling. Microwave for 30 seconds. Stir the crumb topping to prevent the sugar from burning. Cook one more cycle for 30 seconds. Stir crumb topping. Microwave for 20 additional seconds (30 seconds may be too long and the sugar in the crumb topping may start to burn).
Remove content from microwave. Place crumb topping over apple and serve (with a dollop of vanilla ice cream, optional).
Makes 1 individual size serving.
