Preheat the oven to 325 degrees F (165 degrees C).

Whisk sugar, flour, cornstarch, and salt together in a medium saucepan. Stir in water, lemon juice, and lemon zest. Cook over medium-high heat, stirring frequently, until mixture boils. Add butter.

Place egg yolks in a small bowl and gradually whisk in 1/2 cup of the hot sugar mixture. Whisk egg yolk mixture back into the remaining sugar mixture.

Bring mixture to a boil and cook while stirring constantly until thickened. Remove from heat and pour filling into the baked pastry crust.

Beat egg whites in a glass, metal, or ceramic bowl until foamy. Gradually add sugar and beat until stiff peaks form.

Spread meringue over the pie filling, sealing the edges at the crust. Create peaks on the meringue with the back of a spoon if desired.

Bake pie in the preheated oven until meringue is golden brown, about 20 to 25 minutes.