In a medium saucepan, whisk together sugar, flour, cornstarch and salt. Stir in water, lemon juice and lemon zest. Cook over medium-high heat, stirring often, until it comes to a boil. Whisk in the butter. Put egg yolks in a medium bowl and very slowly whisk in ½ cup of the hot sugar mixture (a tiny bit at a time- you want to gradually warm it up, not cook the eggs). Whisk the egg yolk mixture back into the remaining sugar mixture. Bring to a boil and continue to cook, stirring constantly, until thick. Remove from heat. Scrape the filling into the baked pie crust.
PREPARE MERINGUE:
In a large glass or metal bowl, use an electric mixer to beat the egg whites and cream of tartar until foamy. Gradually beat in the sugar (one tablespoon at a time), and continue to beat until stiff peaks form. Spread the meringue over the pie and all the way to the edge of the crust.
BAKE:
Bake for 10 minutes (keep an eye on it), or until the meringue is lightly browned.
Notes
Don't skip adding the cream of tartar in the meringue. It's an acid that stabilizes the egg whites.