whisk together the oats, flour, and cinnamon in a bowl. Make a well in the center. Pour in the maple syrup and butter and stir until all incorporated.
Filling
toss the blueberries with the cornstarch until completely covered.
Transfer the filling to the prepared pan, and sprinkle with the topping. Bake at 350 degrees for 45 minutes or until juice is bubbling in the pan.
Cool to room temperature then refrigerate for at least 3 hours before serving to allow juices to thicken.
Notes
It will keep for up to a week in the refrigerator. It is best with ice cream or cool whip. If you feel it is not sweet enough you can add sugar alternatives such as honey or substitute sweeteners.