For the pastry: Mix the flour, caster sugar and salt together into a bowl. Add the diced butter, rubbing in with your hands until it resembles breadcrumbs. Add the water 1tbsp at a time until the dough starts to come together. Turn out onto a lightly floured worktop, and knead until it comes together to form a uniform dough. Don't handle it too much. Wrap the dough in clingfilm and put in the fridge for 1 hour.
Once chilled, roll the pastry out on a floured work surface into a rectangle about 2-3mm thick. Line the tin with the pastry and then pop back in the fridge for 30 minutes until firm. Then pre-heat the oven to 180C/160C Fan/Gas Mark 4.
Once the pastry is firm, line it with baking paper and fill with baking beans. Then bake for 20 minutes. Once the pastry is baked, remove from the oven and remove the beans and baking paper. Then return to the oven and bake for another 5 minutes until the pastry is dry. Leave to cool completely.
For the curd: Place the diced butter into a medium heat-proof bowl and set aside. Then using a vegetable peeler, peel the coloured zest from the lemons in strips leaving the white pith behind.
Combine the zest, lemon juice, sugar and eggs in a medium saucepan and place over a medium heat whisking constantly. Bring the mixture to a simmer, then remove from the heat and using a sieve strain over the butter. Ensure the zest is removed and discarded.
Using two bowls, sieve the mixture back and forth three times ensuring the curd is smooth.
Pour the curd into the cooled tart shell and spread evenly. Then chill in the fridge for at least 2 hours. You can leave the tart in the fridge overnight at this stage.
For the swiss meringue: Combine the egg whites and sugar in a large greasefree heat-proof bowl over a pan of simmering water, ensuring the base of the bowl doesn't touch the water. Whisk every now and then to ensure you don't cook the eggs, and heat until you reach 60C on a food thermometer. Then remove from the heat and using an electric whisk, whisk until you reach stiff peaks.
Put the meringue into a piping bag fitted with a Wilton 104 piping tip and pipe two 'squiggles' down each side of the tart lengthways.