Prepare pan. Preheat the oven to 325°F. Create a parchment sling and place it into a 9x13 baking dish, leaving overhang on the long sides.
Make crust. Add the graham crackers to a food processor and pulse into fine crumbs. Add the lemon zest, melted butter, and sugar, then process until combined.
Bake crust. Press the mixture evenly into the prepared dish and bake at 325°F for 5 minutes.
Start filling. In a medium saucepan over medium heat, whisk together the flour, lemon juice, and sugar until smooth and the sugar dissolves.
Temper eggs. In a large bowl, whisk the eggs just until broken. Slowly whisk the hot lemon mixture into the eggs, a little at a time, until fully combined.
Thicken. Pour the mixture back into the saucepan and cook, whisking frequently, until thickened. Remove from heat.
Bake bars. Pour the lemon filling over the baked crust and gently spread into an even layer. Bake at 325°F for 25 minutes.
Cool and serve. Cool at room temperature for 1 to 2 hours, then refrigerate for 1 to 2 hours or longer. Slice into squares, dust with powdered sugar, and serve.
Notes
Variations. Use gluten free flour and gluten free graham crackers if needed. Lemon juice concentrate may be substituted for lemon juice.
Recipe Tips. Crush crackers in a sealed bag with a rolling pin if needed. Wipe the knife clean between cuts for neat slices.
Storage. Refrigerate covered bars for up to 1 week, best within 3 to 4 days. Freeze airtight for up to 3 months and thaw in the refrigerator before serving.