Preheat your oven to 350°F (175°C). In a mixing bowl, combine graham cracker crumbs, granulated sugar, and melted butter until the mixture resembles wet sand. Press this mixture into a pie dish. Bake for 8-10 minutes until golden brown, then let cool.

In a saucepan, whisk together granulated sugar, cornstarch, fresh lemon juice, lemon zest, and egg yolks. Over medium heat, stir until it thickens, about 7-10 minutes. Stir in the butter until melted and mixed.

Allow the lemon filling to cool at room temperature for about 10 minutes. For quicker cooling, refrigerate but ensure it firms slightly before adding the meringue.

Whip the reserved egg whites until soft peaks form. Gradually add cream of tartar and sugar, beating until stiff peaks form, about 3-5 minutes.

Spread the meringue over the cooled lemon filling, ensuring it touches the crust. Bake at 350°F (175°C) for 10-12 minutes until golden brown. Watch carefully to avoid overbaking.

Let the pie cool on a wire rack. Refrigerate for at least 2 hours before slicing and serving chilled.