Preheat your oven to 350°F (175°C).
Prepare your pie crust and blind bake it for 10 minutes.
In a medium saucepan, combine sugar, cornstarch, and salt.
Gradually whisk in water and cook over medium heat until thickened.
In a separate bowl, whisk egg yolks and temper them by adding a bit of the hot mixture.
Combine the tempered yolks with the saucepan mixture, then stir in lemon juice, zest, and butter.
Pour the lemon filling into the baked crust and spread evenly.
In a clean bowl, beat egg whites until foamy, then add cream of tartar.
Gradually add sugar and beat until stiff peaks form.
Spread the meringue over the lemon filling, sealing the edges, and bake for 10-15 minutes until golden.