Apple Blueberry Pie Bars

blueberries, fresh or frozen (do not thaw)

granulated sugar

orange juice

cornstarch

large Granny Smith apple, peeled and sliced

and 1/2 cups (190g) white whole wheat flour (or mixture of whole wheat and all-purpose) (measured correctly)

(80g) old-fashioned rolled oats (or quick oats)

granulated sugar

baking powder

ground cinnamon

butter, melted