1 For the pastry: sift the flour into a mixing bowl and stir in the salt. Rub in the butter until the mixture resembles breadcrumbs. Gradually add the water, a tablespoon at a time, mixing continuously until the mixture just comes together as a dough. (You may not need to use all of the water). Roll the dough into a ball, then wrap in clingfilm and chill for 1 hour.
2 Preheat the oven to 200°C (180° fan) 400°F gas 6. Grease a pie dish.
3 Roll out the dough on a floured work surface and line the dish. Prick the base of the pastry case several times with a fork. Cover the pastry with a sheet of non-stick baking paper and fill it with rice or dried beans. Bake for 15 minutes, until the pastry is pale golden. Remove from the oven and remove the beans and paper.
4 Reduce the oven temperature to 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 dish.
8 For the meringue topping: whisk the egg whites until firm peaks form. Whisk in 1 tablespoon of sugar, then add the remaining sugar, 1 tablespoon at a time, whisking constantly until stiff and shiny.
9 Spread the meringue topping over the pie filling and bake for 45-50 minutes, until the meringue is light golden and crisp on the outside, but soft underneath. Serve warm or cold.