Step 1: Preheat your oven to 375°F (190°C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides. Lightly spray the parchment paper with cooking spray.
Step 2: In a large bowl, whisk together 2 ¼ cups all-purpose flour, 1 cup granulated sugar, 1 teaspoon baking powder, and ½ teaspoon salt.
Step 3: Add 1 cup (2 sticks) cold unsalted butter, cut into small cubes, to the dry ingredients. Cut the butter into the flour mixture using your fingertips, a pastry blender, or a stand mixer with a paddle attachment until it resembles coarse crumbs with some larger pea-sized pieces.
Step 4: Press about two-thirds of the crumble mixture evenly into the bottom of the prepared baking pan. Use the back of a spoon or your clean hands to firmly compact the crust. Place the pan in the refrigerator while you prepare the filling.
Step 5: In a separate medium bowl, gently combine 4 cups fresh blueberries, ½ cup granulated sugar, 2 tablespoons all-purpose flour, 1 tablespoon fresh lemon juice, and ½ teaspoon lemon zest (if using).
Step 6: Retrieve the baking pan from the refrigerator. Evenly spread the blueberry filling over the chilled crust.
Step 7: Crumble the remaining one-third of the topping mixture evenly over the blueberry filling.
Step 8: Bake for 45-55 minutes, or until the crumble topping is golden brown and the blueberry filling is bubbly around the edges. If the topping browns too quickly, lightly tent with foil.
Step 9: Remove from the oven and place on a wire rack to cool completely for at least 2-3 hours. For best results, chill in the refrigerator for an hour after reaching room temperature. Once cooled, use the parchment paper overhang to lift the bars from the pan and cut into squares or rectangles before serving.