Combine the sugar, cornstarch and salt in a medium saucepan.
Gradually stir in the ½ c, cold water, and lemon juice and stir until smooth.
Add and mix thoroughly the egg yolks and butter to the mixture.
Sir constantly and add 1 ½ c. boiling water.
Bring the mixture to a full boil as you stir gently.
Reduce the heat as it begins to thicken and allow to simmer slowly for 1 minute.
Remove from heat and stir in the lemon zest.
Pour into the baked pie shell and cover with meringue.
Preheat oven to 350F.
Whip egg whites until frothy.
Add cream of tartar.
Whip until they are stiff but not dry. They should stand in peaks.
Beat in slowly, 3 tablespoon sugar being careful not to over beat.
Spread on pie and bake 10 - 15 minutes.
You can use fresh or lemon juice concentrate. But, in my opinion, fresh squeezed lemon juice definitely gives a better flavor than lemon juice concentrate!
When making the meringue, be sure to beat the egg whites until stiff peaks are formed. Then add the sugar slowly while continuing to beat the egg whites. Once mixed well, stop - you don't want to over beat them.