Preheat the Oven: Start by preheating your oven to 175 °C (350 °F). This ensures that your meringue will bake evenly and become golden.

Make the Filling: In a saucepan, combine the water, granulated sugar, cornstarch, salt, lemon juice, and lemon zest. Cook over medium heat while stirring constantly until thickened and bubbling (about 5-7 minutes).

Incorporate the Butter: Remove the saucepan from the heat and immediately add the unsalted butter. Stir until it melts completely and is well combined with the filling mixture.

Tempering the Egg Yolks: In a separate bowl, beat the egg yolks. Pour a small amount of the hot lemon mixture into the yolks to temper them, then add everything back into the saucepan. Cook for an additional 2 minutes, stirring continually.

Fill the Pie Crust: Pour the lemon filling into your pre-baked pie crust and smooth it out evenly.

Make the Meringue: In a clean, dry bowl, beat the egg whites with the cream of tartar until they form stiff peaks. Gradually add the remaining sugar while beating until the meringue is glossy.

Cover and Bake: Spread the meringue over the lemon filling, making sure to seal the edges so it doesn't shrink. Bake for 10-15 minutes, until the meringue is golden brown.

Cool Before Serving: Allow the pie to rest for 2 hours before slicing. This step is essential for the perfect texture!