Method

In a large mixing bowl, combine the flour, icing sugar, salt and lemon zest. Rub the butter into the flour using your fingertips, until flakes have formed. Make a well in the centre and add 2 egg yolks and 1 tablespoon of ice-cold water. Use a dinner knife to incorporate the flour into the liquid and form a rough ball of dough, then use your hands to pat it into a disc. Cover in clingfilm and place in the fridge for 30 minutes.
Once the pastry has chilled, lightly dust a clean work surface with flour. Roll out the pastry to a 3mm thickness – ensure you roll it enough to allow a little pastry to overhang the edge of the tin. Line a 23cm fluted, loose bottomed tart tin or quiche dish with the pastry. Prick the base a few times with a fork and allow to chill in the fridge again for 30 minutes.
Remove the pastry case from the fridge and lay it on a baking sheet. Line with greaseproof paper and fill with baking beans or rice, then blind bake for 15 minutes. Meanwhile, whisk the remaining 1 egg white a little.
Remove the baking beans and parchment, brush the inside of the case with the lightly whisked egg white, then return the pastry case to the oven for a further 5 minutes until lightly golden. Allow the case to cool then trim off the excess pastry with a Y-shaped peeler.
To make the filling, combine the caster sugar and cornflour in a small mixing bowl, then mix to a paste with the lemon juice and limoncello. Transfer to a medium sized saucepan and bring to the boil. Allow to bubble whilst stirring for 1-2 minutes or until thick, then reduce the heat. Stir in 4 egg yolks and lemon zest. Once fully incorporated, stir in the butter until smooth. Pour into the pastry case and allow to set.
Meanwhile, make the Italian meringue. Place the caster sugar in a medium sized saucepan with 75ml water. Bring to the boil then pop in a sugar thermometer.
When the thermometer shows 110C, whisk the egg whites into stiff peaks in the bowl of a free-standing mixer. Once the syrup has reached 120C, remove from the heat immediately and with the mixer still running, pour a steady stream into the egg whites. Keep whisking until the meringue is cool and glossy.
Remove the tart from the tin and place onto a serving plate or board. Spoon the fluffy meringue on top of the filling. Use the back of the spoon to create delicate, flicked peaks. Before serving, use a blowtorch to lightly caramelise the meringue, to add golden speckles of colour.