Pre-heat the oven to 180C/350F/Gas 4.
First make the pastry. Measure the flour and butter into a food processor and blend together until the mixture resembles fine breadcrumbs. Add the icing sugar, egg and one tablespoon of water and whizz again until combined to a ball.
Tip the pastry onto a work surface and roll out to a 3mm thickness. Use the rolling pin to lift the pastry up and transfer it to line a 23cm/9in loose-bottomed flan tin. Be careful not to stretch the pastry as you tuck it into the corners. Cover in cling film and place in the refrigerator to chill for 30 minutes.
Take the pastry-lined tin out of the fridge and trim the excess pastry. Press the top edge of the pastry so that it stands slightly higher than the top of the tin.
Line the pastry case with parchment and fill with baking beans. Bake for about 15 minutes then remove the beans and parchment and return to the oven for a further five minutes.
Remove from the oven and reduce the temperature to 170C/340F/Gas 3½.
For the filling, mix the lemon zest and juice with the cornflour and stir to form a smooth paste. Measure 450ml/16fl oz of water into a pan and bring to the boil. Add the lemon cornflour mixture to the hot water and stir over the heat until the mixture has thickened, then remove from the heat.
In a bowl mix together the sugar and egg yolks and carefully whisk into the lemon mixture in the pan. Stir over a medium heat until thickened. Set aside for a few minutes and then pour into the baked pastry case.
For the meringue, whisk the egg whites in a free-standing mixer until soft peaks form when the whisk is removed. Add the caster sugar a little at a time, still whisking until the meringue is stiff and glossy. Add the cornflour and whisk again.
Spoon on top of the filled pastry case and spread the meringue to completely cover the lemon filling. Then create a swirl on the top of the meringue.
Bake in the oven for about 15 minutes until the filling is completely set and the meringue is lightly golden and crisp.
Serve warm or cold.