Preheat the oven to gas 6, 200°C, fan 180°C.
Dust a work surface with flour, then roll the pastry out large enough to line a 20cm (8in) loose-bottomed tart tin. Leave the excess pastry overhanging and prick the base with a fork. Chill for 20 minutes until firm. Put the tin onto a baking sheet, line the pastry with nonstick baking paper and baking beans, then bake for 15 minutes. Remove the beans and paper then bake for a further 15 minutes or until golden. When cool, trim the edges with a serrated knife.
For the filling, combine the cornflour, sugar, lemon zest and juice in a saucepan and whisk until smooth. Add the milk, then cook over a low heat, whisking all the time, until the mixture thickens and boils. (It may look lumpy at first but keep whisking and it will come together.) Remove from the heat, then whisk in the butter, followed by the yolks and the whole egg. Set aside.
For the meringue, put the egg whites into a large, clean bowl and mix with an electric hand whisk until stiff peaks form. Add the sugar a third at a time, bringing back to stiff peaks after each addition, until thick and shiny. Whisk in the cornflour until combined.
Bring the lemon filling back to the boil and stir until thickened, then pour it into the pastry case and level the surface. Spoon on the meringue, starting at the edge and working it into the middle. Use a palette knife to make a smooth join with the pastry edge, then swirl the top.
Bake for 20 minutes until golden. Allow to cool in the tin for at least 1 hour, then turn out and serve warm or cold.
To avoid soggy pastry, we’ve baked the base completely before adding the filling. The meringue will prevent the edges burning during final baking.
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If serving vulnerable groups, elderly people, babies, toddlers, pregnant women and people who are unwell, cook until the whites and yolks are solid.