Preheat oven to 350F.

Combine the sugar, flour, salt and water in a heavy saucepan. Stir constantly over medium-high heat until mixture boils. Boil, stirring constantly and vigorously, for 1 minute. (A long-handled spoon to stir with really helps here to get you back from the heat.) Remove from heat.

Slightly beat the egg yolks in a bowl with a fork. Mix half the boiled mixture with the egg yolks. Then put the egg yolk mixture back into the pan with the boiled ingredients, and cook for 1 more minute, stirring constantly and, again, vigorously. Remove from heat, and add the butter.

Add the lemon zest and lemon juice. Stir to mix thoroughly.

Pour into cooled, baked 9-inch pie crust and top with meringue (recipes follow), sealing meringue to edge of pastry. Bake in a 350F oven 12 to 15 minutes or until nicely browned.

Meringue

Beat the egg whites at high speed with an electric mixer until soft peaks form. With mixer running, add the cream of tartar, then gradually add the sugar, a tablespoon at a time, and beat until stiff peaks form. Beat in the vanilla.

Pile atop pie, and bake at 350F for 12 to 15 minutes, or until lightly browned.

Be sure to seal the meringue to the pastry edge when spreading it on your pie. To minimize "weeping", spread the meringue on the pie filling while the filling is hot. Also, remember that meringue pies cut better with a wet knife blade.