Preheat oven to 350 degrees F (175 degrees C).

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.

Meringue Topping:
In a large glass or metal bowl, whip egg whites until foamy. Add sugar gradually, and continue to whip until stiff peaks form. Spread meringue over pie, sealing the edges at the crust.

Bake in preheated oven for 10 minutes, or until meringue is golden brown.

Extra notes

I find you get a better Meringue if the eggs are bought to room temperature before whisking the whites.
Do yourself a favour, get a power egg beater. I did these with a hand whisk. My arm was ready to fall off by the time I was finished.

Those of you with a sharp eye will have noticed that the recipe calls for TWO lemons and there are THREE in the photo.
I like a really tart pie so I zest two and use the juice from three lemons.

I got this basic recipe off the web some years ago. I can't remember where, so a thank you to the original poster.