Separate the eggs. Finely grate the zest of lemons and then juice them (you’ll need 100 ml strained juice). Bring a large saucepan of water to the boil. Put the egg yolks, zest, juice and castor sugar into a large stainless steel bowl and set this over the boiling water, then whisk until pale and thick. Remove the bowl and set aside.
Soften the gelatine in a little cold water for 5 minutes, then squeeze out any excess moisture and dissolve the leaves in the hot water. Whisk this into the egg mixture and refrigerate until cold and just starting to set.
Whish the cream until soft peaks form, then fold this through the cold sabayon. Refrigerate until just starting to set. Whisk the egg whites until soft peaks form (be careful not to over whip them or the texture of the final dessert will be crumble rather than soft and light) and fold these through too.
Spoon the mixture into individual dishes or a large bowl and refrigerate until set, about 3 hours.