Preheat the oven to 350F degrees. Line an 8x8 baking pan with parchment paper leaving an overhang around the edges. Or line with aluminium foil leaving an overhang around the edges and grease with non-stick cooking spray.
Oatmeal Base & Topping
In a medium bowl whisk together the flour, oatmeal, granulated sugar, ¼ cup brown sugar, cinnamon, and salt. Gently stir in the melted butter & egg yolk. Note: if the mixture feels very buttery, optionally add in 1 extra tablespoon of flour.
Remove about 1 cup (not packed) of the mixture and mix with the additional 1 tablespoon of brown sugar.
Press the remaining mixture into the bottom of the prepared pan.
In a medium bowl stir together the berries, sugar, lemon juice and cornstarch until the berries are coated.
Assembly and Baking
Spoon the berry mixture over top of of the oatmeal base.
Crumble the reserved oatmeal mixture over top of the blueberry layer. I like to form crumbles by squeezing it in my fist.
Bake in the preheated oven for about 45-55 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 closer to 50-60 minutes.
Cool the bars fully (while keeping them in the pan) until they're room temperature (at least 3 hours) before slicing.
To slice the bars, lift them out of the pan using the overhang of the parchment paper/aluminum foil. Place on a cutting board and slice with a large, sharp knife.
Bars can be served cold from the fridge, room temperature, or warmed up in the microwave. Optionally, add a scoop of ice cream on top of each.
Make sure to use old-fashioned oats, also known as large rolled oats. Do not use quick oats, instant oats or steel-cut oats for this recipe.
If the oatmeal layer seems very damp/buttery, optionally add in 1-2 additional tablespoons of flour as needed.
This recipe is designed for an 8x8 inch (20x20 cm) pan. There is not quite enough batter for a 9x9 inch pan - therefore, I do not recommend using that pan size.
To double the recipe, double all the ingredients and bake in a 9x13 inch pan. The bake time will be about the same.
Nutrition information is based on 1 bar, assuming the recipe is sliced into 9 equal pieces. It is an estimate only.
Store bars in an airtight container in the fridge for up to 4 days. Bars can be frozen for up to 2 months and thawed in the fridge.