For the shortcrust pastry:

1. Preheat the oven: Preheat your oven to 180°C (350°F) to prepare for blind baking the pastry.
2. Make the pastry: In a mixing bowl, rub the butter into the flour using your fingertips until the mixture resembles breadcrumbs. Stir in the icing sugar and egg yolk, then add cold water, a little at a time, until the dough comes together.
3. Chill the dough: Wrap the dough in cling film and chill in the fridge for 20-30 minutes.
4. Roll out the pastry: Roll out the chilled dough on a lightly floured surface until it’s large enough to line a 9-inch pie dish. Trim the edges and prick the base with a fork.
5. Blind bake the pastry: Line the pastry with parchment paper and fill with baking beans. Bake for 15 minutes, then remove the paper and beans and bake for another 5 minutes until golden. Set aside to cool.

For the lemon filling:

1. Prepare the lemon mixture: In a saucepan, mix the cornflour with 100ml of the water to form a smooth paste. Add the lemon juice, zest, and the remaining water, and stir over medium heat until thickened.
2. Add the egg yolks: Remove the mixture from heat and whisk in the egg yolks, caster sugar, and butter until smooth and glossy.
3. Cook the filling: Return the pan to low heat, stirring constantly until the mixture thickens into a smooth lemon curd. Set aside to cool slightly, then pour into the baked pastry shell.

For the meringue topping:

1. Whip the egg whites: In a clean, dry mixing bowl, whisk the egg whites using an electric hand mixer until they form soft peaks.
2. Add the sugar: Gradually add the caster sugar, one spoonful at a time, while continuing to whisk until the meringue forms stiff, glossy peaks.
3. Top the pie: Spread the meringue evenly over the lemon filling, ensuring it reaches the edges of the pastry. Use the back of a spoon to create swirls and peaks.

Bake the pie:

1. Final bake: Return the pie to the oven and bake for 15-20 minutes, or until the meringue is golden brown. Allow the pie to cool before serving.