Using a fine grater, grate the zest from the lemons into a medium bowl. Squeeze and strain the juice and keep to one side.
Put the gelatine in a small bowl and mix with the lemon juice. Allow to “sponge” (swell) for 10 mins.
Meanwhile, add the yolks and sugar to the lemon zest and whisk until light and pale-lemon coloured.
Set the bowl of sponged gelatine in a pan of simmering water and leave until melted and clear. Stir into the yolk mixture.
Beat the Quark (with the Limoncello, if using) until smooth. Then fold the egg yolk mixture into it.
Now whisk the egg whites to soft peaks and gently fold into the mix. Pour into a pretty dish or individual glass bowls. Level the surface and chill overnight or for 3-6 hours.
Serve decorated with extra Quark and chopped lemon zest.