Preheat your oven to 350°F or 177°C. Grab a 9-inch square pan and line it with parchment paper. Set aside.

In a medium bowl, whisk in 1 cup of oats, the all-purpose flour, brown sugar, baking powder, cinnamon, salt, and lemon zest.

Mix the melted butter with the dry ingredients until combined, resembling a moist crumbly sand texture, free of dry spots. You will have around 3 cups (or 520 g) of crust mixture.

Pour ⅔ of the crust dough (around 2 cups) into the pan. Press the mixture down the bottom of the pan using your hands, a spatula, a large spoon, or anything with a flat bottom. Bake for 10 minutes. Then, take it out and let it cool.

In a saucepan, stir the blueberries, cornstarch, sugar, and lemon juice over medium heat for a good 2 or 3 minutes until the sugar and cornstarch dissolve completely and the blueberries become evenly wet. Take the saucepan out of the heat to toss in the lemon zest.

Spread the filling evenly on the warm crust. Mix the remaining 1 cup of crust mixture with 2 tablespoons of whole rolled oats and sprinkle it over the filling. Use a spatula or a large spoon and slightly press the crumble texture so it sticks to the filling.

Bake for 45 to 55 minutes until the top turns golden brown and the filling starts bubbling.

Let the pan cool completely on a wire rack. Lift the bars up, cut them into squares, and enjoy!