Ingredients

Step by step

1. Rub the butter into the flour, adding a good pinch of salt, then stir in the icing sugar. Mix the egg yolk with 2 tablespoons of cold water and stir in with a knife, adding extra water if necessary (or if not using the egg yolk), then bring the dough into a ball by hand. Flatten into a disc, wrap and chill for at least 30 minutes.
2. Roll out the pastry on a floured surface and line a deep 23cm flan tin, pressing in well and trimming off the excess pastry. Chill for 30 minutes. Preheat the oven to 200°C, fan 180°C, gas 6.
3. Fill the pastry case with a sheet of crumpled baking paper and baking beans and bake blind for 15 minutes on a baking tray. Remove the paper, press the pastry down gently and return to the oven to bake for 7-10 minutes until pale golden and crisp. Remove and cool.
4. Meanwhile, finely grate the zest from 4 lemons (see tip below, if you want to make a candied lemon zest garnish with the rest of the zest). Put the grated zest in a saucepan with the cornflour and caster sugar. Squeeze the juice from the lemons – you need 175ml for a good balance of sweet and sharp, so measure this into a measuring jug, then make up to 375ml with water. Gradually add to the pan, stirring or whisking until smooth. Bring to a simmer, stirring until it thickens, then cook for 2 minutes to cook out the rawness of the cornflour. Remove from the heat and stir in the egg yolks, followed by the butter. Press through a sieve to remove the zest and any eggy bits that may have formed, then smooth into the tart case and leave to cool completely. It will only come about half-way up the pastry case, leaving enough depth to hold the meringue topping.
5. When ready to bake, preheat the oven to 150°C, fan 130°C, gas 2. In a sparklingly clean bowl, whisk the egg whites to stiff peaks, starting with the mixer on slow, then gradually increasing the speed (starting slowly gives a more stable meringue). Once you have stiff peaks, add the cornflour, then add the sugar a spoonful at a time, whisking well between each one. The meringue should be shiny, stiff and not feel grainy. Spoon on top of the cooled filling, making sure you seal to the edges of the pastry case. Swirl and peak it as you wish; we used a small palette knife to create slight ridges in the meringue. Bake, still on the tray, for 25-30 minutes until the meringue is set and crisp on the outside, but not too browned. Leave to settle and cool for at least 30 minutes before serving either warm or at room temperature.
   Tip

   To make a candied lemon garnish, pare the zest from 2 lemons using a veg peeler. Dissolve 50g caster sugar in 50ml water in a small saucepan. Shred the lemon zest (scrape off any white pith first) and simmer in the syrup for 5 minutes. Set aside to cool, then drain before use.