Sift the flour and salt into a mixing bowl.
Add the butter to the flour and then rub between your fingertips until a fine breadcrumb consistency is achieved.
Add the sugar to the flour and butter. Using your hands, mix to a firm dough with the egg yolk and a splash of cold water.
Wrap the dough in cling film and allow to rest in the refrigerator for 30 minutes before using.
Preheat the oven to 190C/375F/Gas 5 and grease a 22cm/9in fluted loose-bottomed tin. Place the tin onto a baking tray.
Roll out the pastry until it's big enough to generously fit the tin, leaving excess pastry falling over the sides of the tin onto the baking tray underneath the tin - don't trim at this stage.
Line the pastry with greaseproof paper and fill with baking beans. Now trim the excess pastry using a sharp knife. Bake the pastry case blind for about 15-20 minutes or until the pastry is pale golden and dried out - remove the paper and beans for the last five minutes. Lower the oven heat to 150C/300F/Gas 2.
Meanwhile, for the lemon curd, mix together the sugar, cornflour and enough water to make a paste, in a large bowl.
Bring 50ml/2fl oz water and the lemon zest to the boil in a small pan.
Gradually pour the hot liquid onto the cornflour and sugar, whisking all the time until smooth.
Beat in the egg yolks, lemon juice and butter. Return to the pan.
Cook over a low heat, stirring all the time, until thickened. Pour into the baked blind pastry case and leave to cool slightly.
For the meringue, whisk the egg whites in a large bowl with an electric whisk until they form stiff peaks. Whisk in the caster sugar, a spoonful at a time, whisking well and at a high speed between each addition. Transfer the meringue into a piping bag (with a plain nozzle) and pipe the meringue on top of the lemon curd.
Bake for about 35-45 minutes until the meringue is crisp on the outside and soft and marshmallow-like underneath.
Serve warm or cold in slices.