Preheat oven to 350°F (177°C). Line a 9-inch square baking pan (I love this one and this one) with parchment paper, leaving an overhang on the sides to easily lift the bars out of the pan. Set aside.

Whisk the flour, 1 cup (85g) oats, brown sugar, baking powder, lemon zest, cinnamon, and salt together in a medium bowl. Add the melted butter and stir until the mixture resembles moist crumbly sand and no dry spots remain. You will have a little more than 3 cups of crust mixture, or about 520g. Press 2/3 of the mixture (about 2 cups/345g—doesn’t have to be exact) into the prepared baking pan. Press it into the pan with your hands, a large spoon or spatula, or the bottom of a measuring cup to form a crust. Bake for 10 minutes, and then remove from the oven to slightly cool.

Place the blueberries, sugar, cornstarch, and lemon juice in a saucepan over medium heat on the stove and stir gently to combine. Cook and stir for 2–3 minutes until the cornstarch and sugar begin to dissolve and the berries look evenly wet. Remove from heat and stir in the lemon zest.

Pour/spread the filling over the warm crust. Stir 2 Tablespoons oats into the remaining crumble mixture, then sprinkle it all over the filling, and lightly press it down with the back of a large spoon or flat spatula.

Bake for about 45–55 minutes, or until the top is lightly browned and the blueberry filling is bubbling around the edges.

Remove from the oven and allow the bars to cool completely in the pan set on a wire rack. Feel free to cover and refrigerate the cooled bars for up to 1 day before cutting into squares.

Lift the cooled bars out of the pan using the overhang on the sides and cut into squares.

Cover and store leftover blueberry pie bars at room temperature for up to 2 days or in the refrigerator for up to 1 week.