Preheat your oven to 350°F (175°C).
Bake the pie crust according to package instructions until golden brown.
In a saucepan, combine sugar, cornstarch, and salt.
Gradually stir in water over medium heat, cooking until thickened.
In a bowl, whisk egg yolks, then temper with the heated mixture.
Add lemon juice, zest, and butter to the tempered egg yolks, stirring well.
Pour the lemon filling into the baked pie crust.
In a clean bowl, beat the egg whites and cream of tartar until soft peaks form.
Gradually add sugar, beating until stiff peaks form and glossy.
Spread the meringue over the lemon filling, sealing the edges.
Bake for an additional 10-15 minutes until meringue is golden.