Preheat oven to 350 degrees
Medium saucepan, whisk together 1 cup of sugar, flour, cornstarch, and salt.
Stir in water, lemon juice and lemon zest.
Cook over medium-high heat, stirring frequently until mixture comes to a boil. Stir in butter. Place egg yolks in a small bowl and gradually whisk in 1/2 cup of hot sugar mixture.
Whisk egg yolk mixture back into remaining sugar mixture. Bring to a boil and continue to cook while stirring constantly until thick.
Remove from heat.
Pour filling into baked pastry shell.
Meringue: whip egg whites until foamy. Add sugar gradually, and continue to whip until stiff peaks form. Spread meringue over pie.
Bake in preheated oven for 10 minutes.