Put the egg whites and sugar in the bowl you'll use to whip them up later.
Place the bowl in a bain marie on medium heat without the water touching the base of the bowl.
Use a whisk to move the egg whites every couple of minutes, trying also to scrape the sides of the bowl.
Remove it from the heat when you put your finger inside and don't feel any granules of sugar left. Be particularly careful to check the bottom of the bowl.
Whisk on high speed until the egg whites come to room temperature.
Dissolve the cornflour in the lime juice and mix into the egg whites. This will make it marshmallowy at the centre.
Cut a paper to the size of the base of your 42x28cm baking tray and stick it in place with a bit of the meringue in the corners.
Spread out the meringue and take it to the oven at 100C for 45min.
Let it cool down completely before filling.