Preheat the oven to 200C/400F/Gas 6. Grease a 20cm/8in fluted flan tin.
To make the pastry, place the flour, sugar and the butter in a food processor and pulse until the mixture resembles fine breadcrumbs. With the motor running, gradually add the beaten egg and blend until the mixture forms a ball. Do not over blend or the pastry will be tough.
Tip onto a floured board, knead briefly, then roll out to a 5mm/¼in thickness, turning the pastry and flouring the surface regularly. Use it to a line the prepared flan tin. Trim the edges neatly, prick the base lightly with a fork and chill the pastry for 30 minutes.
While the pastry is resting, make the filling. Put the cornflour in a small bowl and mix with enough of the cold water to make a thin paste then set aside. Pour the remaining water into a large saucepan and add the sugar, lemon zest and juice. (You should have around 225ml/8fl oz lemon juice.) Heat gently until the sugar dissolves then bring to the boil. Reduce the heat slightly and quickly stir in the cornflour mixture - the sauce should thicken immediately.
Cook over a low heat for three minutes, stirring until thickened and glossy. Remove from the heat and cool for five minutes. Whisk the egg yolks with the whole egg until smooth, then whisk vigorously into the sauce. Set aside to cool for 25 minutes.
Put the pastry case on a sturdy baking tray, line with crumpled baking parchment and fill with baking beans. Bake blind for 15 minutes then carefully take out of the oven and remove the paper and beans. Return to the oven for a further 3-4 minutes until the surface of the pastry is dry. Remove from the oven and reduce the temperature to 150C/300F/Gas 2.
For the meringue topping, whisk the egg whites in a large bowl until stiff then gradually whisk in half the sugar. Add the vanilla extract and whisk in the remaining sugar.
Stir the cooled lemon filling and pour into the pastry case. Cover very gently with large dessertspoonfuls of the meringue topping, starting at the sides then working your way into the middle, and gently swirl the top. Bake for 25 minutes or until the meringue is set and very lightly browned. Leave to cool before removing from the tin.