1. Prepare Your Baking Pan and Oven: Begin by preheating your oven to 350 F (175 C). Line a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. This small step makes cleanup and serving much simpler.

2. Assemble the Shortbread Crust: In a medium bowl, combine 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, and 1/2 teaspoon salt. Add the cold, cubed butter and use a pastry blender, your fingertips, or a fork to cut the butter into the flour mixture until it resembles coarse crumbs. The mixture should hold together when pressed.

3. Press and Partially Bake the Crust: Press the crust mixture evenly into the bottom of your prepared baking pan. Use the bottom of a flat glass or your hands to ensure it’s compact and smooth. Bake for 15-20 minutes, or until the edges are lightly golden. This partial bake helps create a firm base for your lemon filling.

4. Prepare the Lemon-Blueberry Filling: While the crust is baking, whisk together the 4 large eggs, 1 1/2 cups granulated sugar, 1/4 cup all-purpose flour, 1/2 teaspoon baking powder, 1/2 cup fresh lemon juice, and 1 tablespoon lemon zest in a large bowl until smooth. Gently fold in the 1 1/2 cups of fresh blueberries, being careful not to crush them too much.

5. Combine Crust and Filling for Final Bake: Once the crust is partially baked, remove it from the oven. Carefully pour the lemon-blueberry filling evenly over the warm crust. Return the pan to the oven and bake for an additional 20-25 minutes. The bars are done when the center is set and no longer jiggles significantly when gently shaken, and the edges are lightly golden.

6. Cool Completely and Dust: Remove the pan from the oven and place it on a wire rack to cool completely. This is a critical step; do not try to cut the bars while they are warm, as they will be too soft and fall apart. Once fully cooled (this can take 1-2 hours), use the parchment paper overhang to lift the entire slab of bars out of the pan. Dust generously with powdered sugar, if desired, before slicing into squares.