In a medium saucepan, whisk together sugar, flour, cornstarch, and salt. Gradually whisk in water until smooth
Cook over medium heat, stirring constantly, until the mixture comes to a boil and thickens
Remove from heat and stir in lemon juice, lemon zest, and butter until well combined
Gradually whisk some of the hot mixture into the beaten egg yolks, then return everything to the saucepan. Cook and stir for 2 more minutes, then remove from heat
Preheat oven to 375°F (190°C). Bake the pie crust according to package instructions until lightly golden, then set aside to cool
In a clean bowl, beat egg whites on medium speed until soft peaks form. Gradually add sugar, one tablespoon at a time, beating on high until stiff peaks form
Pour the hot lemon filling into the pre-baked pie crust and spread evenly
Spread meringue over the hot filling, sealing it to the edges. Create peaks with a spoon
Bake for 10–12 minutes until the meringue is lightly golden
Let the pie cool completely before slicing and serving