Preheat the oven to 350F degrees. Line an 8×8 baking pan with aluminium foil and make sure there is an overlay on the sides so it will easy to pick it up. Spray lightly with non-stick cooking spray.
In a medium bowl whisk together the flour, oatmeal, granulated sugar, 1/4 cup brown sugar, and salt. Gently stir in the melted butter and egg yolk. Set aside 1 cup of the mixture.
Press the remaining mixture into the bottom of the prepared pan. Make sure to make it nice and even.
In another medium bowl mix together all the ingredients for the blueberry layer. Spoon over top of the base oatmeal cookie mixture in the pan.
To the reserved 1 cup of oatmeal mixture, stir in the 1 additional tablespoon of brown sugar.
Crumble the rest of the oatmeal mixture over top of the blueberry layer.
Bake in the preheated oven for about 40-45 minutes or until you see the blueberries bubbling and the top crumble turning golden brown. If you used frozen berries, the bake time will be 50-55 minutes.
Allow to cool fully to room temperature (at least 3 hours) before cutting into the bars.