Use double boiler: Blend sugar, salt, flour and cornstarch. Stir in 1 cup cold water. Heat the remaining 2 cups water before adding. Cook mixture slowly over low heat, stirring constantly until thick and clear. This takes at least 45 minutes in a double boiler.
Add well beaten egg yolks, cook 1 minute longer. Add butter, lemon juice and rind. Cool and pour into crust, baked. Top with meringue and bake as any other meringue topped pie.