Prepare the lemon filling on the stovetop by combining sugar, cornstarch, water, lemon juice, lemon zest, and butter in a saucepan. Cook until thickened.
In a clean bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add sugar, beating until stiff peaks form.
Pour the prepared lemon filling into a pre-baked pie crust.
Spoon the meringue over the lemon filling, ensuring to spread it to the edges to seal.
Bake the pie in the preheated oven for 10-15 minutes or until the meringue is golden brown.
Once baked, allow the pie to cool completely on a wire rack.
For optimal taste and texture, serve the lemon meringue pie chilled or at room temperature.