Preheat oven to 375°F (190°C). Place the pie crust in a pie dish. Bake for about 10 minutes until lightly golden. Let it cool completely before adding the filling.

In a saucepan, mix sugar and cornstarch. Add water slowly while whisking. Cook over medium heat and stir constantly until the mixture thickens.

In a small bowl, beat the egg yolks. Slowly add a few spoonfuls of hot mixture to the yolks. This warms them gently before adding them back.

Pour the egg mixture back into the saucepan. Add butter, lemon juice and lemon zest. Cook for two minutes until thick and smooth.

Pour the hot lemon filling into the baked pie crust. Spread it evenly using a spatula. The filling should be smooth and glossy.

Place egg whites in a clean bowl. Add cream of tartar. Beat with a mixer until soft peaks form.

Slowly add sugar while beating the egg whites. Continue mixing until stiff peaks form. The meringue should look shiny and fluffy.

Spread the meringue over the warm lemon filling. Make sure the meringue touches the crust edges. This prevents shrinking during baking.

Place the pie back in the oven. Bake for about 10 minutes until the meringue turns golden brown on top.

Remove the pie from the oven. Let it cool at room temperature for one hour before slicing. This helps the filling set properly.