Preheat oven to 350. To make lemon filling, mix sugar, flour, cornstarch and salt in a saucepan. Stir in water, lemon juice and lemon zest. Cook over medium heat stirring constantly until mixture comes to a boil. Stir in butter.
Mix about 1/2 cup of hot mixture into the egg yolks, beat and stir back into saucepan. Bring to a boil and continue to cook until thick (about a minute). Be sure to continue to stir or the sugar will burn on the bottom of the pan. Pour filling into baked pastry shell.
In large glass or copper bowl whip egg whites and sugar until stiff peaks form. Spread meringue over warm pie, sealing the edges at the crust. Bake in preheated oven for 10 minutes or until meringue is golden brown. Cool completely.