In a food processor, add the flour, powder sugar, and salt, and mix until combined.
Add the cold cubed butter, and blend until there are mini pea-sized pieces of butter.
If you don't have a food processor, you can do it by hand, mixing the cold butter into the flour mixture. Make sure that your hands are cold.
Then, whisk the egg in a small bowl, add the egg gradually while the food processor is on low speed, and mix until a dough forms. Try not to overmix the dough.
Cover the dough with plastic wrap and let it chill in the fridge for at least an hour.
Make the lemon curd filling
Into a saucepan over medium low heat, add the lemon juice, sugar, eggs, egg yolks, and lemon zest, and mix with a whisk constantly.
Keep whisking until the mixture thickens for about 5-8 minutes. When the mixture thickens, add the butter and mix with a whisk until the butter is melted.
Transfer the mixture to a fine-mesh strainer set over and bowl and strain the mixture from any unnecessary lumps. This will give you a perfect and smooth filling.
Cover the bowl tightly with plastic wrap and let the mixture cool for about 20-30 minutes at room temperature and then in the fridge while you bake the crust.
Transfer the dough onto a clean floured surface roll it with a rolling pin into a circle moving the dough constantly to ensure it doesn’t stick to the surface and transfer it into a 9-inch tart pan.
Cut off the extra edges. Make little holes with a fork and freeze for 20 minutes.
Bake in a 180c (350f) preheated oven for about 15-20 minutes or until the crust is a light golden brown color. Remove from the oven and allow it to cool for about 10 minutes.
Pour the curd into the tart crust and smooth it out with an offset spatula. Bake the tart in a 180c (350f) preheated oven for about 10-12 minutes or until the edges are set and the middle is still juggly.
Remove the tart from the oven and allow it to cool at room temperature for 15 minutes, then transfer it to the fridge to set for at least 4 hours. Chilling it overnight is optimal.
Make the Swiss meringue
Start by combining egg whites and sugar in a heatproof bowl. Place the bowl over a pot of simmering water, creating a double boiler.
Whisk the mixture constantly while heating until it reaches 160°F (71°C) or feels smooth and not grainy when rubbed between your fingers.
Once the desired temperature is reached, remove the bowl from heat and transfer the mixture to a stand mixer fitted with a whisk attachment.
Beat on high speed for about 5-7 minutes, or until stiff, glossy peaks form and the meringue has cooled to room temperature.
Top the tart with the meringue, spread it with an offset spatula, and make a wave pattern. Don't overdo it so you don’t deflate the meringue.
Blowtorch the meringue to finish it off. Serve immediately or store in the fridge for up to 4 days.
Notes
Don't let the lemon curd come to a boil! This will cook the eggs and leave you with scrambled eggs. Make sure that the lemon curd mixture cooks over medium-low heat.
Use COLD butterfor the crust, I suggest cutting the butter into cubes and then placing it in the fridge, so when you are ready to use it, it will be very cold.
The star of your tart is the lemon, so select fresh, juicy lemons with bright yellow skin. Organic lemons are ideal, as you'll be using both the juice and zest for maximum flavor.
Freeze the crust for at least 20 minutes before baking it. This will keep your crust from collapsing while baking.
Zest before juicing. Always zest your lemons before juicing them. This makes the process much easier and ensures you get the most out of each lemon.
Strain your lemon juice. After juicing your lemons, strain the juice to remove any pulp or seeds. This will result in a smoother, more refined lemon curd filling.
Let the pastry dough rest in the fridge for at least 1 hour before handling it. This will make it easier to roll out the crust and handle it.
If the crust puffs a bit while baking, press it down using a cup.
Constantly whisk while making the curd! It is really important if you want to get a smooth and silky curd.
Use an easy-release tart pan. I use an easy-release tart pan for all of our tart recipes. This makes sure that your tart will be easy to release from the pan.
Chill thoroughly before serving. Allow your tart to cool completely at room temperature, then refrigerate for at least 5 hours. This helps the filling set properly and enhances the flavors.