01 - Preheat oven to 350°F. Set a rack in the center position.
02 - Combine graham cracker crumbs, sugar, and melted butter in a mixing bowl until the mixture resembles wet sand. Press firmly and evenly into the bottom and up the sides of a 9-inch pie pan.
03 - Bake the crust for 8 minutes until lightly toasted and fragrant. Remove from the oven and set aside to cool while preparing the filling.
04 - Whisk together granulated sugar, cornstarch, salt, and lemon zest in a medium saucepan. Gradually whisk in the lemon juice, egg yolks, and water until completely smooth and no lumps remain.
05 - Place the saucepan over medium heat and whisk constantly for 6 to 8 minutes until the mixture thickens noticeably and comes to a gentle boil. Remove from heat immediately and whisk in the butter until fully incorporated and glossy.
06 - Pour the hot lemon filling directly into the baked crust, spreading it into an even layer with a spatula.
07 - In a spotlessly clean bowl, beat egg whites and cream of tartar with an electric mixer on medium speed until soft peaks form. Increase speed to high and add the sugar in a slow, steady stream. Continue beating until stiff, glossy peaks hold their shape.
08 - Spread the meringue over the hot filling, making sure to seal the edges directly against the crust to prevent shrinking or weeping. Create decorative swirls or peaks with the back of a spoon.
09 - Bake for 12 to 15 minutes until the meringue peaks are golden brown and lightly toasted.
10 - Cool the pie completely at room temperature, then refrigerate for at least 2 hours before slicing to allow the filling to set properly.