Rinse the lemons. Grate the zest and squeeze the juice.
Separate the eggs, putting the yolks into a heatproof bowl. Beat the egg yolks with the caster sugar, until creamy and fluffy.
Pour the lemon juice through a small strainer into the mixture. Add the zest and mix well.
Put a small amount of water into a saucepan and place over a low heat.
When the water starts to simmer, sit the bowl with the lemon cream in it to make a bain-marie. Stirring continuously, cook the lemon cream for 7-8 minutes until it thickens and coats the back of a spoon.
Meanwhile, fill a larger bowl with cold water. Remove the bowl with the lemon cream from the heat and plunge it into the larger bowl. Leave to cool completely.
Whisk the egg whites with the icing sugar and a pinch of salt until they form soft peaks.
Fold the egg whites into the lemon cream using a metal spoon.
If you would like your mousse to have a more intense lemon colour, fold in a few drops of natural food colouring.
Divide the mousse into 4 individual dishes and refrigerate for 30 minutes.
Decorate with sprigs of mints before serving.
This super easy pud it's zesty and refreshing end to a meal. Gluten free too.