01 - Preheat oven to 350°F. Position rack in the center of the oven.
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. Remove from the oven and set aside to cool while preparing the filling.
04 - In a medium saucepan, whisk together sugar, cornstarch, salt, and lemon zest. Gradually whisk in the lemon juice, egg yolks, and water until completely smooth.
05 - Place the saucepan over medium heat and whisk constantly for 6 to 8 minutes, until the mixture thickens significantly and comes to a gentle boil. Remove from heat and whisk in the butter until fully melted and incorporated.
06 - Pour the hot lemon filling directly into the prepared crust, spreading it into an even layer.
07 - In a clean, grease-free mixing bowl, beat egg whites and cream of tartar with an electric mixer until soft peaks form. Gradually add sugar, one tablespoon at a time, beating continuously until stiff, glossy peaks hold their shape.
08 - Spread the meringue over the hot filling, making sure to seal the edges against the crust to prevent shrinking. Bake for 12 to 15 minutes, until the meringue peaks are golden brown.
09 - Allow the pie to cool completely at room temperature, then refrigerate for at least 2 hours before slicing and serving.