1 Heat the oven to 200°C (180° fan) 400°F gas 6. Grease a 20 cm|8" pie dish.
2 Roll out the pastry on a floured surface and line the tin. Prick the base with a fork, then place a circle of non-stick baking paper over the base of the pastry and cover with ceramic baking beans (or uncooked rice).
3 Bake 'blind' for 18-20 minutes until lightly golden. Remove from the oven and remove the baking beans or rice and the baking paper.
4 Reduce the oven temperature 150°C (130° fan) 300°F gas 2.
5 For the filling: put the sugar and the lemon zest and juice into a pan.
6 Mix the egg yolk with the cornflour mixture, then add to the pan with the boiling water. Bring to a boil, stirring, then simmer over a low heat, stirring constantly, until thickened. Remove from the heat and allow to cool.
7 Pour the cooled filling into the pastry case in the tin.
8 For the meringue topping: whisk the egg whites until stiff peaks form. Whisk in 1 tablespoon of sugar, then add the remaining sugar, 1 tablespoon at a time, whisking constantly.
9 Spread the meringue topping over the pie filling, piling it up in the centre and bake for 45-50 minutes, until the meringue is light golden and crisp on the outside, but soft underneath. Serve warm or cold.