Prep the pan and oven. Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving overhang on the long sides for easy lifting. Lightly grease the parchment.

Make the shortbread crust. In a bowl, combine 1 1/2 cups flour and 1/2 cup sugar. Stir in the melted butter until the mixture looks like damp sand and holds together when pressed.

Press and pre-bake. Press the crust mixture firmly and evenly into the prepared pan. Use the bottom of a measuring cup to smooth. Bake for 18–20 minutes, or until the edges are lightly golden and the top looks set.

Whisk the filling. While the crust bakes, whisk the eggs, remaining 1 1/4 cups sugar, 1/2 cup flour, lemon juice, and lemon zest until smooth. Avoid over-whisking to prevent excess bubbles.

Pour over hot crust. Give the filling one last stir. Pull the crust from the oven and immediately pour the filling over the hot crust. This helps the layers bond and set cleanly.

Bake until set. Return to the oven for 18–22 minutes, or until the center looks set and no longer jiggles when you nudge the pan. The top may have a thin, pale layer—that’s normal.

Cool completely. Let the pan cool at room temperature for at least 1 hour, then chill for another hour for the cleanest slices. Don’t rush this step.

Slice and serve. Lift the slab out using the parchment overhang. Dust with powdered sugar if you like. Use a sharp knife, wiping between cuts, to slice into bars.