Preheat your oven to 350°F (175°C). If using a pre-made pie crust, follow the package instructions for baking. If making from scratch, roll the dough into a 9-inch circle and fit into a pie pan, then prick the bottom with a fork. Bake for about 10-12 minutes until lightly golden and cool.

In a medium saucepan, whisk together the sugar, cornstarch, and salt. Gradually stir in the water. Bring to a simmer over medium heat while constantly whisking until thickened. Remove from heat and slowly mix in the beaten egg yolks, lemon juice, lemon zest, and butter until smooth.

Pour the lemon filling into your prepared crust, spreading it evenly.

In a clean mixing bowl, beat the egg whites with the cream of tartar until soft peaks form. Gradually add the sugar and continue to beat until stiff and glossy peaks form.

Spread the meringue over the lemon filling, sealing the edges to avoid shrinking during baking.

Bake for 10-15 minutes, or until the meringue is lightly golden. Remove from the oven, let it cool for a bit, then refrigerate until set.