• Grab 4 eggs and separate the egg yolks from the egg whites and set aside.

Meringue:

• In a small saucepan, whisk together cornstarch and water. Cook over medium heat, stirring constantly until thickened. Remove from heat and set aside.
• In a small bowl, stir together cream of tartar and sugar. In a large bowl, beat the egg whites with electric mixers until frothy. 
• With the mixers running, add the sugar mixture, a spoonful at a time, until fully incorporated. Continue beating until soft peaks. Add the thickened cornstarch mixture while beating, a spoonful at a time. Mix until stiff peaks form, about 1-2 minutes. Set aside.
• Preheat oven to 350 degrees F.

Lemon Filling:

• In a medium saucepan, whisk together 1 cup sugar, flour, cornstarch, and salt. Stir in water, lemon juice and lemon zest. Cook over medium-high heat, stirring frequently, until mixture comes to a boil. Remove from heat and stir in butter. 
• Beat the egg yolks with a fork. Add a spoonful of the hot saucepan mixture into the egg yolk mixture and whisk together to temper the eggs (gradually warm them).  Repeat, with a few more spoonfuls. Whisk the egg yolk mixture back into remaining sugar mixture in the pot. 
• Bring to a gently boil and continue to cook while stirring constantly, until thick. Remove from heat.

Assemble:

• Pour hot lemon filling into baked pie shell. Immediately add meringue on top, covering every surface, spreading it around all the way to the edge of the pie so there are not gaps between the pie crust and meringue. Use the back of your spoon to create decorative peaks in the meringue.
• Bake in preheated oven for 10 minutes or until the meringue is lightly browned. 
• Transfer to a wire rack and cool to room temperature, about 2 hours. This pie is best served the same day.
• Store leftover lemon meringue pie covered in the refrigerator for 2-3 days.