Whisk sugar, flour, cornstarch, and salt together in a medium saucepan until evenly combined.

Slowly stir in water, lemon juice, and lemon zest until smooth, then place over medium-high heat.

Cook the mixture, stirring frequently, until it thickens and reaches a gentle boil, then stir in the butter until melted.

Gradually whisk a small amount of the hot filling into the beaten egg yolks to temper them.

Pour the warmed egg mixture back into the saucepan and cook, stirring constantly, until thick and glossy.

Remove from heat and spread the lemon filling evenly into the prepared pie crust.

Beat egg whites in a clean bowl until foamy, then slowly add sugar while beating until stiff peaks form.

Immediately spread the meringue over the hot filling, sealing it completely to the crust edges.

Use the back of a spoon to create soft peaks, then bake until the meringue is lightly golden, about 20–25 minutes.

Remove from the oven and allow the pie to cool before slicing and serving.