Prepare lemon filling

From lemons, grate 1 tablespoon lemon peel and squeeze ¾ cup juice.

In 2-quart saucepan, mix sugar, cornstarch, and salt; stir in water until blended. Cook over medium-high heat, stirring occasionally, until mixture thickens and boils. Boil 1 minute, stirring. Remove from heat.

In small bowl, with wire whisk, whisk egg yolks. Stir in ½ cup hot cornstarch mixture until blended; slowly pour yolk mixture back into cornstarch mixture in saucepan, stirring rapidly to prevent curdling.

Place saucepan over medium-low heat and cook until mixture comes to a gentle boil, stirring constantly. Cook, stirring until filling is very thick 2 to 3 minutes. Remove from heat; stir in butter until melted. Stir in lemon juice and peel; mixture will thin out. Pour into pie shells.

Preheat oven to 400º F.

Prepare meringue topping

In medium bowl, with mixer on high speed, beat egg whites, cream of tartar, and salt until frothy. Gradually sprinkle in sugar, 2 tablespoons at a time, beating until sugar completely dissolves and egg whites stand in stiff, glossy peaks when beaters are lifted.

Spread meringue over warm filling. To keep meringue from shrinking during baking, make sure it touches edge of crust all around. Use back of wooden spoon to make attractive swirl patterns on top of meringue.

Bake until meringue is golden, 6 to 8 minutes. Cool pie complete on wire rack away from drafts. Refrigerate at least 1 hour before serving.