Put the flour and butter in a food processor and pulse together briefly until it resembles breadcrumbs. Add the sugar and briefly pulse, then add the egg yolks and 2-3 tbsp ice cold water until the mix just starts to bind. Tip the pastry on to a lightly floured surface, gather together into a smooth ball and then roll out into a large circle, ½cm thick.
Using a rolling pin, pick up the pastry and drape over a 21 x 3½cm loose-bottom fluted flan tin. Press the pastry into the tin, squeezing together any cracks. Prick the base with a fork and chill for 1 hour.
Put a baking sheet in the oven and preheat to 200°C/fan 180°C/gas mark 6. Line the chilled pastry case with greaseproof paper and fill with baking beans. Bake the pastry case on the baking sheet for 15 minutes. Remove the greaseproof paper and beans and brush the inside of the pastry case with one of the egg whites (you won’t need all of it). Return to the oven and bake for 6-8 minutes more until golden. Remove, then lower the oven to 180°C/fan 160°C/gas mark 4.
Mix 2 tbsp cornflour, 100g sugar and the lemon zest in a medium saucepan. Gradually add the lemon juice, whisking as you go. Place over a medium heat and cook, stirring constantly, for 3-4 minutes until thickened and smooth. Once the mixture starts to bubble, remove from the heat and beat in the butter until melted. Whisk together 3 egg yolks and the whole egg. Gradually add to the pan, whisking as you go. Place back over a medium heat and continue stirring vigorously for 3-4 minutes, until the mixture thickens. Take off the heat and cover.
Put 5 egg whites in a large bowl. Whisk until soft peaks form, then add the remaining sugar a large spoonful at a time, whisking between each addition. Sieve over 2 tsp cornflour, then give a final whisk until smooth and thick.
Pour the lemon filling into the pastry case. Put large spoonfuls of meringue around the edge of the filling, then spread so it just touches the pastry. Pile the rest of the meringue on to the centre and spread so it touches the surface of the hot filling. Add swirls to the top, then return to the oven for 18-20 minutes until the meringue is crisp and slightly golden in colour. Let the pie sit in the tin for 30 minutes, then remove and leave for 1-2 hours before slicing.