Preheat the oven to 180ºC/160ºC Fan. Line 2 large baking sheets with non-stick baking paper and set aside.
Place the egg whites into the bowl of a stand mixer, making sure it is spotlessly clean and dry. Add a generous pinch of flaky sea salt. Whisk on a low speed until foamy, then gradually increase the speed to medium and continue to whisk until soft peaks form (this will take about 3-4 minutes).
Meanwhile, in a separate bowl, mix together the caster sugar and lemon zest using your fingertips (this releases citrus oils in the zest which makes it more zingy).
Increase the speed of the mixer to medium-high, then gradually add the lemon sugar a spoonful at a time. Make sure that each spoonful is completely whisked in before adding the next. Continue until all the sugar has been added and you have a smooth and shiny meringue that holds its shape when turned upside down. This should take about 5 minutes.
Sieve in the cornflour and drizzle in the lemon juice. Fold to combine.
Lift the non-stick baking paper up from the baking sheets and dab a bit of the meringue on the corners, then recover with the non-stick baking paper (this will help stop it from slipping).
Using 2 spoons, scoop equal-sized mounds of the meringue, spaced apart, onto the prepared baking sheets (you should have around 12). Use the back of a spoon to make a little indentation into the centre of each meringue (this will leave space to add your lemon curd).
Place the trays into the oven, then reduce the temperature to 150ºC/130ºC Fan and bake for 30 minutes. Turn the oven off and leave the meringues inside for another 30 minutes. Then remove from the oven and leave to cool completely (they should be crisp on the outside but deliciously chewy on the inside).
Meanwhile, make the lemon curd (optional): In a large heat-proof glass or ceramic bowl, mix together the sugar and lemon zest using your fingertips. Rub together until clumpy and totally mixed. Add the egg yolks and, using a rubber spatula or silicone whisk, whisk until thickened, pale and slightly foamy.
In a large saucepan, heat the lemon juice over medium-high heat until it just comes to a boil. Remove from the heat and gradually pour over the foamy lemon-sugar egg whites, whisking constantly until all the lemon juice has been added.
Return the lemon-egg mixture to the saucepan and cook over low heat, stirring constantly until thickened enough to coat the back of a spoon - this should take around 5 minutes. Make sure you keep it over a gentle, low heat and stir constantly - if you leave it or let it boil, you will end up with a peculiar lemon scrambled egg yolk situation (not what we are looking for!).
Once it has thickened, remove from the heat and gradually add the butter. Stirring to ensure it is fully melted and incorporated. Pour into a jar and leave to cool completely before covering and chilling in the fridge.
To serve: place the cooled meringues onto a serving plate and top with dollops of lightly whipped cream, a spoonful of lemon curd and fresh fruit. Alternatively, build into a tower and dollop over the cream before drizzling over the lemon curd. Enjoy!