To make lemon filling: In a medium saucepan, whisk together 1 cup 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.
To make Meringue: In a large glass or metal bowl ( I always put my metal in the freezer to get really cold), whip egg whites until foamy. Add sugar gradually, and continue to whip until stiff peaks form, I always cream of tartar too..
Bake in a preheated oven for 10 minutes, or until meringue is golden brown.