Preheat the oven to 180C/350F/Gas 4. In a food processor, blend the plain flour and unsalted butter together until the mixture looks like fine breadcrumbs. Add the confectioners' sugar, beaten egg, and one tablespoon of water. Process again until the dough comes together in a ball. Tip the dough onto a work surface and roll it out to about 3mm thickness. Carefully lift and use it to line a 23cm/9in loose-bottomed flan tin, making sure not to stretch the pastry. Cover with cling film and chill in the refrigerator for 30 minutes. I like to handle the pastry gently in this stage to ensure a crisp, non-shrunken crust.

Take the chilled pastry-lined tin from the fridge, trim any excess pastry, and gently press the top edge so it stands slightly higher than the tin. Line the case with parchment and fill with baking beans. Bake in the preheated oven for 15 minutes. Remove the beans and parchment, then return the case to the oven for another 5 minutes. Remove from the oven and reduce the temperature to 170C/340F/Gas 3½.

Mix the zest and juice from the lemons with 2 1/4 oz cornstarch until it forms a smooth paste. Measure out 16 fl oz water and bring it to a boil in a pan. Add the lemon-cornstarch mixture to the hot water, stirring constantly over the heat until the mixture thickens. Remove from heat. In a bowl, whisk together 9 oz superfine sugar and 6 egg yolks. Gradually whisk this into the lemon mixture in the pan. Stir over medium heat until the filling thickens further. Set aside for a few minutes, then pour the filling into the baked pastry case from Step 2.

In a stand mixer, whisk the egg whites until soft peaks form. Gradually add the superfine sugar, whisking continuously until the meringue is stiff and glossy. Sprinkle in 2 tsp cornstarch and whisk again to incorporate. I find adding cornstarch helps stabilize the meringue so it holds up beautifully when baked.

Spoon the meringue on top of the lemon filling (from Step 3) in the baked pastry case. Spread the meringue to entirely cover the filling and use the back of a spoon to create swirls on top. Bake in the oven (now set at 170C/340F/Gas 3½) for about 15 minutes, or until the meringue is lightly golden and crisp and the filling is completely set.

Allow the lemon meringue pie to cool completely before cutting into slices, or serve it very slightly warm if you prefer. For a cleaner slice, I like to chill the pie in the fridge for an hour before serving.