In a separate bowl, combine the brown sugar, cinnamon, nutmeg, and cornstarch. Then stir it into the blueberry mixture and cook for an additional 2 minutes, stirring constantly. Once the mixture has thickened and resembles pie filling, remove from heat. To make the oatmeal crumble, combine all ingredients in a large bowl and use a pastry cutter or your fingers to mix the ingredients together, squishing the cubes of butter and creating a coarse crumble.