Mix the sugar and cornstarch in a medium saucepan. Slowly stir in the water and cook the mixture over moderate heat, stirring constantly until it thickens and boils for 1 minute. Remove the saucepan from the heat.
To temper the eggs, slowly add half of the hot mixture to the egg yolks in a mixing bowl, whisking constantly until incorporated. Then return the egg mixture to the saucepan and whisk again to incorporate.
Place the saucepan back on the heat and cook until the mixture thickens about 1 minute. Remove the saucepan from the heat.
Add the butter, lemon juice, and lemon zest, stir until incorporated, and pour the hot filling into the baked pie shell.
To make the meringue: In a large bowl, beat the egg whites with an electric mixer using the whisk attachment until stiff peaks form. Add the cream of tartar and sugar to the meringue mixture.
Spread the meringue over the lemon filling, sealing to the edge, and bake for 8 to 10 minutes, until delicately browned. Let cool at room temperature away from drafts.
You will need just one pie shell for this recipe if you want to make a homemade pie shell, most recipes make two shells. So you can half the recipe if desired.
You may want to blind bake your pie shell before you start the lemon filling. Tempering eggs takes a bit of time and you will want to give that portion of the cooking your undivided attention.
This is my Grandmother's recipe. She was known for being a wonderful pie maker. I dearly miss her pies. This recipe is by Ethel Eynard. Jefferson City, MO. 1998.