• Preheat the oven to 400ºF.

Filling

• In a saucepan, combine the sugar, the flour, the cornstarch and the salt. Whisk to combine. Stir in the water and the lemon juice. Cook over medium heat, whisking or stirring constantly, until the mixture is thickened. It should be thick enough to coat the back of a spoon.
  1 cup granulated sugar, 1/4 cup all-purpose flour, 3 tablespoons cornstarch, 1/4 teaspoon salt, 2 cups water, 1/4 cup fresh lemon juice
• Place the egg yolks in a small bowl and beat slightly. While whisking, stir in a bit of the hot mixture, whisking constantly to prevent scrambling the eggs. Pour this egg mixture back into the saucepan and continue to cook, whisking constantly, for 1 minute.
  3 egg yolks
• Remove the mixture from the heat and add in the butter and lemon zest. Stir to combine, then pour into the prepared pie crust.
  1 tablespoon unsalted butter, 1 tablespoon lemon zest, 1 9-inch pie crust, cooked and cooled

Meringue

• To make the meringue, beat the egg whites in a large bowl until they are frothy. (This is definitely easiest done in a stand mixer or with a hand mixer.) Start adding in the 6 tablespoons of sugar, about 1 tablespoon at at time. Continue to mix until stiff, marshmallow-like peaks form.
  3 egg whites, 6 tablespoons granulated sugar
• Carefully spread the meringue over the hot filling, making sure to go all the way to the edges of the pie to touch the crust.

Bake

• Bake in the preheated oven for about 5 minutes, or until the peaks are browned.

Refrigerate

• Let the pie cool at room temperature until completely cool. If not serving right away, refrigerate for 3-6 hours before serving.