Preheat oven to 325°F (165°C).
Whisk sugar, flour, cornstarch, and salt in saucepan. Add water, lemon juice, and zest. Cook until boiling.
Stir in butter, then temper yolks with some hot mixture. Whisk back into saucepan until thick.
Pour filling into baked crust.
Beat egg whites until foamy, add sugar gradually, whip to stiff peaks.
Spread meringue over hot filling, sealing edges.
Bake 20–25 minutes until golden.
Cool at room temperature, then refrigerate before slicing.
