Preheat oven to 200°C and grease a 20cm flan tin.
Pulse flour, sugar, and butter in a food processor until breadcrumb texture forms.
Add beaten egg and blend until mixture forms a ball.
Roll pastry to 5mm thickness and line the flan tin.
Prick the base with a fork and chill for 30 minutes.
Mix cornflour with some cold water to create a thin paste.
Boil remaining water, sugar, lemon zest, and 225ml of juice.
Stir in cornflour paste and cook over low heat until glossy.
Cool for five minutes then whisk in egg yolks and whole egg.
Bake the pastry case blind for 15 minutes using parchment and beans.
Remove beans and bake for 4 minutes until dry then reduce oven to 150°C.
Whisk egg whites to stiff peaks and gradually incorporate sugar and vanilla.
Pour lemon filling into the case and top with spoonfuls of meringue.
Bake for 25 minutes until the meringue is set and lightly browned.