In a microwave-safe bowl, mix together the blueberries and cornstarch. Cover with a paper towel and microwave for 2 minutes, watching the mix carefully toward the end of the time so it doesn't bubble over.
In a small bowl, combine the oats, brown sugar, flour, and cinnamon. Cut the butter into small pieces and use your hands to press it into the oat mixture, tossing the ingredients together until the mixture is crumbly. Sprinkle the topping over the blueberries.
Microwave for one minute, stir, then microwave for one more minute. Let sit at room temperature for a minute or two before diving in.
Notes
This makes one (1) serving. You can double the recipe and use larger bowls to make 2 servings.