To make the pastry: put the flour, sugar and salt into a food processor and pulse a few times to mix it together. Cut the butter into 1cm cubes, and add it to the flour. Process the mixture until it resembles fine breadcrumbs, taking care not to over mix it, then add the egg and switch the motor on again until the pastry starts to come together. Once it starts to form one large ball in the machine, tip it out onto a floured surface and bring it together quickly with your hands. (If the pastry is still dry, add 1 tbsp water and pulse a few times to bring it together.)
Roll the pastry out to around 0.6cm thick. Use it to line your pie tin, pressing it firmly into the flutes, then trim the edges, prick the base with a fork, and refrigerate for 1 hour.
Preheat the oven to 200C (180C fan). After an hour, line the pie dish with foil and fill with baking beans or dry rice or lentils. Blind bake the pie case for 15 minutes, then remove the beans and foil, brush with egg wash, and bake for another 10-15 minutes, until cooked through and golden brown.
To make the pie filling: put the sugar and water into a large saucepan and heat gently, stirring until the sugar has dissolved. Don’t let it come to a boil - or if it does, let it cool some before the next steps.
Remove from the heat and sprinkle over the powdered gelatine. Use a whisk to combine and dissolve the gelatine, returning the pan to the heat for a moment if it doesn’t dissolve fully. Once totally combined, add the lemon essence and citric acid, and whisk through.
Let the mixture cool, then put the pie case, still in it’s tin, into the fridge. Carefully pour the liquid filling into the pie case and leave it to set in the fridge, for 5-6 hours, or overnight.
To make the meringue: put the caster sugar and water into a small saucepan and stir until the sugar has melted, then bring to the boil. Leave the saucepan without stirring, until it reaches a fast boil. Use a sugar or digital thermometer to check the temperature of the sugar syrup - it needs to reach 120C.
Meanwhile, put the egg whites and lemon juice into the bowl of a stand mixer and whisk until it reaches soft peaks.
Once the sugar has reached 120C, with the stand mixer still running, slowly pour the sugar syrup into the whipped egg whites. Keep whisking until the meringue has formed soft peaks and is cool, then switch off the mixer.
Serving the pie: carefully transfer the pie from its tin onto a serving board or plate. For a traditional looking pie with a surprise clear centre, pile spoonfuls of the meringue on top of the pie and use the back of a spoon to create billowing cloud shapes. Use a blow torch to brown the meringue, starting slowly and taking care not to burn it. Serve within an hour or two of adding the meringue.