It is always a good idea to have everything measured out ahead of time when making a pie.
In a medium sauce pan combine the sugar, cornstarch, salt, and water. Mix well. Bring to a boil. Once boiling, reduce heat to allow to simmer. Stir constantly. Boil for 5 minutes.
Temper the eggs with the hot mixture. Then pour the tempered eggs into the sauce pan and cook for 3-5 more minutes. Stir constantly.
Once you get the mixture to trace, remove from heat and add the butter, lemon juice and zest.
Place the pie filling into a the refrigerator with a piece of plastic wrap resting directly on top of the filling. This will keep the filling from getting a skin on top.
Once the pie shell and the pie filling are cooled, pour the filling into the crust. Keep covered in the refrigerator until you are ready to serve. Top with whipped cream.