Prepare the Oven and Baking Dish: Preheat your oven to 375°F. Lightly grease a 9×13-inch baking dish or line it with parchment paper, leaving some overhang on the sides to make removing the bars easier.

Make the Crumble Mixture: In a large mixing bowl, whisk together the flour, sugar, baking powder, salt, and lemon zest. Add the cold, cubed butter and egg. Using a pastry cutter or fork, work everything together until the mixture resembles coarse, crumbly dough. You can also use a food processor, pulsing just until the ingredients come together without overmixing.

Form the Bottom Crust: Set aside half of the crumble mixture for the topping. Press the remaining half evenly into the bottom of your prepared baking dish, making sure to create a firm, even layer.

Prepare the Blueberry Filling: In a separate bowl, combine the sugar, cornstarch, and fresh lemon juice. Add the blueberries and gently toss until the berries are evenly coated.

Assemble the Bars: Spread the blueberry filling evenly over the prepared crust. Sprinkle the reserved crumble mixture over the blueberries, distributing it across the entire surface.

Bake Until Golden: Place the baking dish in the preheated oven and bake for approximately 45 minutes, or until the crumble topping turns lightly golden brown and the blueberry filling is bubbling.

Cool and Serve: Allow the bars to cool completely in the baking dish before slicing. For cleaner cuts and a firmer texture, refrigerate them for a few hours before serving. Cut into squares and enjoy!