Preheat your oven to 350°F (175°C). Lightly grease a 13x9-inch baking pan with butter. For even easier removal, you can line the pan with parchment paper, leaving an overhang on the sides.
In a large mixing bowl, combine the 2 cups flour, 1/2 cup sugar, and salt. Add the cold, cubed butter. Using a pastry blender or your fingertips, cut the butter into the dry ingredients until the mixture resembles coarse, lumpy crumbs. Press this mixture firmly and evenly into the bottom of your prepared pan.
Place the pan in the preheated oven and bake for about 20 minutes, or until the edges turn lightly golden.
While the crust is baking, prepare the filling. In a medium bowl, whisk together the 1 1/2 cups sugar and 1/4 cup flour. Crack in the 4 large eggs and whisk thoroughly until the mixture is smooth. Stir in the fresh lemon zest and lemon juice until well combined.
Once the crust is out of the oven, immediately pour the lemon filling evenly over the hot crust. Return the pan to the oven and bake for another 20 minutes, or until the filling is set (it should have a slight jiggle in the very center, but not be liquid).
Let the bars cool completely at room temperature on a wire rack. Then, transfer them to the refrigerator to chill for a minimum of 2 hours. Chilling helps the bars set perfectly for clean slices. Before serving, generously dust the top with powdered sugar, then cut into neat squares and enjoy your refreshing, healthy treat!