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.
In a small bowl, beat the egg yolks. Add a spoonful of the hot sugar mixture into the egg yolk mixture and whisk together. (The idea is to temper the egg mixture by adding a little bit of the hot mixture at a time so that the eggs yolks don't start to cook).
Repeat, adding a few more spoonfuls at a time. Whisk the egg yolk mixture back into remaining sugar mixture in the pot.
Bring to a boil and continue to cook while stirring constantly, until thick. Remove from heat. Pour filling into baked pie shell.
For the Meringue:
Preheat oven to 350 degrees F.
In a small saucepan, whisk together cornstarch and water.
Cook over medium heat, stirring constantly as the mixture thickens, until it begins to simmer. Remove from heat and set aside.
In a small bowl stir together cream of tarter and sugar. In a large bowl beat the egg whites with electric mixers until frothy.
With the mixtures running, add the sugar, a spoonful at a time, until fully incorporated. Continue beating until soft peaks.
Add the cornstarch mixture while beating, a spoonful at a time. Mix until stiff peaks form, about 1-2 minutes.
Pour the meringue onto the hot lemon filling in the pie shell. Spoon the meringue around the edge of the pie first, and then add spoonfuls to the center until the entire surface is covered.
Make sure the meringue is touching the outer pie crust so that it doesn't shrink away while baking. 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. This pie is best served the same day.
Store leftover lemon meringue pie in the refrigerator for 2-3 days, covered with aluminum foil.
*Fresh eggs work the best for the meringue to whip properly.Adapted from All Recipes