Roll out the ready-made pie crust so that it fits into a 20cm loose-bottomed pie dish. Line the dish with the pastry, then place in the fridge to chill for 30 minutes.
Preheat the oven to 170°C while the pastry chills. Line the pastry with a piece of parchment paper, then weight it down with dried beans. Bake for 15 minutes, then remove the paper and beans and return to the oven for a further 5–10 minutes, until it’s light golden and slightly crisp. Cool the pastry on a wire rack.
Meanwhile, zest the lemons directly into a medium-sized pot. Juice the lemons and measure the amount – you’re aiming for 250ml of juice. If you’re short, top up the juice with water to get 250ml. Pour the juice into the pot, then add the egg yolks, sugar, cornflour and a pinch of salt. Put the pot over a medium heat and whisk until smooth, then add the butter. Continue stirring for 5–10 minutes, until the butter has melted and the lemon curd has become creamy and thick. Don’t raise the heat too high and don’t walk away from the curd, or the eggs might scramble. Remove the pot from the heat and allow to cool slightly, then pour the curd into the pastry case. Chill in the fridge for at least 30 minutes.
To make the meringue, place the egg whites in a spotlessly clean, dry bowl. Whisk until stiff peaks form. Continue to whisk and add the sugar spoonful by spoonful, until the meringue is glossy and stiff. Finally, whisk in the cornflour.
Dollop the meringue on top of the chilled lemon curd, spreading it right to the edges. Use an offset spatula to make neat swirls in the meringue, or use a spoon to make more rustic peaks all over. Bake for about 25 minutes, until the meringue is golden, set and crisp. Allow to cool on a wire rack before slicing.