Step 1: Preheat your oven to 350°F (175°C) and line an 8x8-inch baking dish with parchment paper, leaving an overhang on the sides for easy removal.
Step 2: In a food processor, combine the flour, 1/2 cup granulated sugar, cold cubed butter, and salt. Pulse until the mixture resembles coarse crumbs. Alternatively, cut butter into dry ingredients using a pastry blender or your fingertips.
Step 3: Press the crust mixture evenly into the bottom of the prepared baking dish.
Step 4: Bake the crust for about 15 minutes, or until lightly golden. Let it cool slightly while you prepare the lemon curd.
Step 5: In a medium saucepan, combine the lemon juice, 1 cup granulated sugar, eggs, and lemon zest. Whisk until fully combined.
Step 6: Cook the mixture over medium heat, stirring constantly, until it thickens and coats the back of a spoon (about 8-10 minutes). Do not boil.
Step 7: Remove the saucepan from the heat and stir in the 1/2 cup cubed butter until fully melted and combined.
Step 8: Pour the lemon curd evenly over the slightly cooled crust. Set aside.
Step 9: In a large, clean bowl, beat the egg whites and cream of tartar with an electric mixer on medium-high speed until soft peaks form.
Step 10: Gradually add the remaining 1/2 cup granulated sugar, continuing to beat until stiff, glossy peaks form.
Step 11: Gently fold in the fresh blueberries until just combined.
Step 12: Carefully spread the blueberry meringue evenly over the lemon curd layer.
Step 13: Return the baking dish to the oven and bake for about 15-20 minutes, or until the meringue is golden brown and set.
Step 14: Let the meringue bars cool completely in the baking dish on a wire rack. Cooling thoroughly is crucial before cutting to ensure clean slices.
Step 15: Once cooled, lift the bars out of the dish using the parchment paper overhang. Optional: Dust with powdered sugar and garnish with fresh lemon slices.
Step 16: Cut the meringue bars into squares and serve. Enjoy your delicious and refreshing Blueberry Lemon Meringue Bars!