FILLING: Mix the cornstarch with a little bit of the water just to make a slurry. Put the remaining water into a saucepan. Add the lemon juice, zest and cornstarch. (PS -- If you don't have a microplane for zesting, a cheese grater would work ok. Microplanes are great, though, so maybe think about adding it to your kitchen!) Bring to a boil while stirring. Cook for 2 minutes. You want the cornstarch to reach its full thickening power. Remove from the heat and allow to cool a little bit -- not completely.
Add the 5 tbsp sugar. SLOWLY add the egg yolks while constantly whisking. If you add them too quickly without tempering them they will cook! No one likes lemon scrambled egg pie.
Pour into the pie shell.
MERINGUE TOPPING: 1. Put the egg whites and a pinch of cream of tartar either into a mixing bowl or into the bowl of your stand mixer. Beat the egg whites until they are thick and have soft peaks. Once it is thick enough, slowly add the sugar while still beating. Beat for a minute or two until it forms stiff peaks.
Add the meringue onto the top of the filling.
Bake the pie at 300 for 40 minutes.
Remove from the oven and allow to cool completely. Once it is cooled, it is best if you allow it to chill in the fridge for a bit so the filling can set.
Enjoy! (Rest assured that even a flat meringue still makes a good pie.)