If you like, you can strain the lemon curd through a fine-mesh sieve before pouring it on the crust—this will give it a smoother mouthfeel.
Make the shortbread crust: In the bowl of a stand mixer fitted with the paddle attachment, combine the ingredients and mix on medium speed until a smooth dough forms, 3 to 4 minutes. Scrape down the bowl as needed. Press the dough evenly into the prepared pan. Bake until just barely golden brown around the edges, 20 to 25 minutes.
Remove the crust from the oven and lower the oven temperature to 325 degrees F. Cool the crust completely in the pan.
While the crust is cooling, make the lemon filling: Combine the sugar and fl our in a medium bowl. Add the eggs and whisk to form a paste. Add the lemon juice and zest and stir to combine. Pour onto the crust.
Bake until the filling is set, 15 to 20 minutes.
Allow to cool completely in the pan before cutting.
Cut into bars and dust lightly with confectioners' sugar.
Lime Bars: Replace the lemon juice and zest with equal amounts of lime juice and zest.