TO PREPARE A BOTTOM LAYER AND A CRUMBLE TOPPING -Start by preheating the oven to 350 F.  Then, line an 8 inch square pan with parchment paper and set aside.  

Next, combine all purpose flour, rolled oats, granulated sugar and light brown sugar in a large dish. Then, add melted butter and stir to combine. You will get a sandy mixture with crumble pieces. You should set aside 1 cup of the mixture that will be used as a crumble topping in the end. The rest of the mixture should be pressed on the bottom of the pan that has already been prepared.

TO PREPARE BLUEBERRY FILLING - start by combining blueberries, granulated sugar, lemon juice, lemon zest and cornstarch in a medium dish. 

Now, spread the mixture over the bottom layer and sprinkle 1 cup of oatmeal crumble mixture. It should be baked for 45 minutes or until crumble topping becomes golden.

After taking it out of the oven, leave it in the baking pan for 20 minutes and then transfer to a wire rack to cool completely, at a room temperature.