Preheat the oven to 180C/160C Fan/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 1 tablespoon water and briefly pulse until combined to form 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. 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. Use a fork to lightly prick the pastry base all over.

Line the pastry case with baking paper and fill with baking beans. Bake for 15 minutes. Remove the beans and paper and return to the oven for a further 5 minutes, or until the pastry is very lightly cooked. Remove from the oven and set aside.

Reduce the oven temperature to 170C/150C Fan/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 water into a pan and bring to the boil. Add the lemon cornflour mixture to the hot water and stir continuously over the heat until the mixture is thick and smooth. 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. Pour into the pastry case.

For the meringue, whisk the egg whites in a freestanding mixer until soft peaks form. Slowly add the caster sugar a little at a time, continuously whisking until all the sugar is incorporated and the meringue is very stiff and glossy. Add the cornflour and whisk again.

Spoon the meringue on top of the filled pastry case and spread to completely cover the lemon filling. Create swirls on the top of the meringue using the back of a fork.

Bake in the oven for about 15 minutes until the meringue is lightly golden and crisp.

Allow to cool a little before carefully removing from the tin. Serve in slices while it's still very slightly warm.