Preheat the oven to 350°F (180°C) with an oven rack in the center. Lightly spray a 13×9-inch (33x23cm) pan with nonstick spray. Line the pan with parchment paper, letting the excess extend over all sides of pan.
Make the graham cracker crust: In a medium bowl, stir the crumbs, sugar, and salt together to combine. Stir in the melted butter and mix to combine. The crust should easily hold together in clumps when you press it together between your fingers.
Press the crumb mixture evenly into the bottom of the prepared pan. Use a dough tamper or the base of a small dry measuring cup to help press the crust in evenly and pack it down a bit in the process.
Transfer the crust to the oven and bake until it begins to visibly brown at the edges, 12 to 15 minutes. The filling will be added to the crust right after baking, so get the ingredients ready but don’t mix it until just before using. Leave oven on.
Make the lemon filling: In a medium bowl, whisk the sugar, flour, and salt together to combine. Add the lemon juice, and whisk vigorously to combine. In a medium bowl, whisk the eggs thoroughly and then add to the other ingredients and whisk well to combine. Pour the filling mixture onto the warm crust.
Bake until the filling is visibly set and an instant-read thermometer inserted in the center registers 170°F (77°C), 22 to 26 minutes. Transfer the pan to a cooling rack to cool completely to room temperature, at least 1 hour, and then transfer to the refrigerator and chill until firm, at least 2 hours, or up to overnight.
When you’re ready to finish the bars, use the excess parchment paper handles to lift the entire bar out of the pan and transfer to a cutting board. Peel away any attached parchment paper.
Just before you’re ready to serve, make the meringue topping: Bring a medium pot filled with about 2 inches (5cm) of water to a simmer over medium-low heat. Set a medium bowl over the pot, add all the ingredients, and whisk to combine. Then heat, whisking constantly, until the mixture reaches 160°F (71°C) on a thermometer.
Remove the bowl from the heat, and transfer to the bowl of a stand mixer fitted with the whisk attachment. Whip the mixture on medium-high speed until it reaches stiff peaks, 6 to 8 minutes.
Pile the meringue on top of the cooled bars. Spread the meringue out to the edges, but leave the surface swoopy in texture. Use a kitchen torch to toast the surface of the meringue.
Cut the bar into 3 even rows across the shorter side, and 4 even rows across the longer side to make 12 pieces. (For the cleanest slices, wipe your knife clean after each cut.) Serve immediately, or cover and refrigerate for up to 8 hours before serving.
