Set oven to 350°F (175°C).
In a saucepan, whisk sugar, flour, cornstarch, and salt. Gradually stir in water, lemon juice, and zest.
Cook over medium heat until thick and bubbly. Stir in butter.
Slowly whisk hot mixture into egg yolks, then return to saucepan. Cook 2 minutes more. Pour into pre-baked crust.
Beat egg whites until foamy. Gradually add sugar and beat until stiff peaks form. Spread over hot lemon filling, sealing edges.
Bake 10–12 minutes, until meringue is golden.
Cool at room temperature, then chill at least 2 hours before serving.