Preheat the oven to 350 degrees. Line an 8x8" baking dish with parchment paper or foil. Place the flour, oats, 1/3 cup sugar, cinnamon and salt in a medium bowl and stir well to combine.

Add the cold cubes of butter to the dry ingredients. Using a pastry cutter, fork or your hands, work the butter into the dry ingredients until the mixture resembles pea-sized crumbles.

Place the blueberries, 1/2 cup sugar, lemon zest and juice, and cornstarch in a medium bowl and stir until combined.

Press 2/3 of the oat crumble into the bottom of the prepared pan.

Pour the blueberry mixture evenly over the crumble.

Sprinkle the remaining 1/3 of the crumble over the blueberry mixture.

Bake for about 50 minutes until the center is bubbly, covering with foil after 30 minutes if the crumble is browning too quickly. Cool the blueberry crumble bars completely on a wire rack for a minimum of 3 hours or overnight until set. Cut into bars and enjoy!