In a saucepan, mix the cornstarch, sugar, and salt.
Gradually add hot water and cook over direct heat, stirring continuously until the mixture becomes thick and clear, about 8 minutes.
Remove from heat.
Beat the egg yolks together and gradually stir 1/4 cup of the cornstarch mixture into the yolks.
Pour this mixture back into the saucepan and cook at low heat, stirring continuously, for about 6 minutes.
Remove from heat and add the lemon juice, peel, and butter. Allow this mixture to cool somewhat before pouring into the pie shell.
To make the meringue, beat the egg whites until soft peaks form.
Gradually add the sugar and cream of tartar and continue beating until the meringue holds in firm glossy peaks.
Spread meringue evenly over the pie filling.
Bake at 350º F for 15 to 20 minutes, until golden brown.