Pre-heat the oven to 180°C.
Blend flour and butter in a food processor until breadcrumbs form, then add icing sugar, egg, and 15ml water to form a ball.
Roll pastry to 3mm thickness on a work surface and line a 23cm loose-bottomed flan tin.
Cover the pastry in cling film and chill in the refrigerator for 30 minutes.
Trim excess pastry so it stands slightly higher than the tin edge.
Line the pastry with parchment, fill with baking beans, and bake for 15 minutes.
Remove beans and parchment and bake for an additional 5 minutes.
Remove from oven and reduce temperature to 170°C.
Mix lemon zest, juice, and cornflour into a paste and stir into 450ml of boiling water until thickened.
Whisk sugar and egg yolks together and stir into the lemon mixture over medium heat until thickened.
Pour the lemon filling into the baked pastry case.
Whisk egg whites to soft peaks, then gradually add caster sugar and cornflour until stiff and glossy.
Spoon meringue over the filling, sealing the edges and creating a swirl on top.
Bake for 15 minutes until the filling is set and the meringue is golden.