FOR THE FILLING:
A pie that's as pretty as it is delicious, the lemon meringue dates back to the mid-1800s. Lemon custards go back much farther, as does meringue. But it took a few hundred years for them to meet in a pastry crust. Make this on a dry, cool day; meringue doesn't fare well in hot, humid weather.
To make the filling, combine the sugar, 1-½ cups water, and salt in the top of a double boiler over high heat. Bring to a boil. Mix the cornstarch with the remaining ⅓ cup water and add to the boiling mixture slowly, stirring constantly. Cook until thickened and clear. Remove from the heat and cool five minutes. Stir in the egg yolks and lemon juice. Return the mixture to the heat and stir until it begins to bubble. Remove from the heat and stir in the butter and zest. Cover and cool until lukewarm.
FOR THE MERINGUE:
Note: If you're uncomfortable using uncooked egg whites, look for pasteurized egg whites, available in the dairy case of many supermarkets.
To make the meringue, combine the egg whites, cream of tartar, and salt in a clean mixing bowl and beat until frothy. Gradually add the sugar, beating until glossy peaks form when you lift the beater.
Preheat the oven to 325°;. Pour the filling into the pie shell. Pile on the meringue, spreading to the edge of the crust. Bake for 15 minutes, or until lightly browned. Cool on a rack for 1 hour. Chill for at least 3 hours before serving. This is best served the day it is made.