Heat the oven to 200°C (400°F) gas 6. Roll the pastry out to a circle 5 mm (1/4 in) thick. Lift into a 20 cm (8 in) flan case and press well into the sides. Trim off the top of the pastry and prick the base. Line the pastry case with foil and beans and bake for 10 minutes. Remove the foil and bake for a further 8-10 minutes.

increase the heat to 220°C (425°F) gas 7. Mix the cornflour, salt, sugar and boiling water in the top pan of a double boiler. Put the pan over direct heat and cook, stirring continuously with a wooden spoon, until the mixture comes to the boil.

Turn down the heat and simmer gently for 15 minutes, stirring from time to time. Meanwhile grate the zest of 1 lemon and squeeze the juice of all 3.

Beat the butter, 90 ml (6 tbsp) lemon juice and the grated lemon zest into the sugar mixture. Stir in the egg yolks and put the pan on the top of a double boiler containing simmering water. Cook, stirring, until thick. Do not allow the mixture to come to the boil or the yolks will curdle. Cool briefly.

Fill the half-baked pastry shell with the mixture.

Using a wire whisk or rotary beater, whisk the egg whites with a pinch of salt until they are stiff. Continue to whisk, adding the caster sugar in 4 batches. Spoon the meringue onto the pie, fluffing up the surface into attractive peaks. Bake in the oven for 15 minutes until golden brown. Serve hot or cold.