Separate the eggs.
Preheat the oven to 200°C (180°C fan, gas 6). Beat the egg whites with the lemon juice and salt until soft peaks form. Gradually add half of the sugar and continue to whisk until stiff.
Mix the flour with the cornflour. Beat the egg yolks with the remaining sugar and 1 tbsp warm water until frothy. Beat in the flour mixture.
Add the beaten egg whites and fold in until combined.
Gently spread out the mixture on a swiss roll tin lined with baking paper. Bake for 10-12 minutes on the middle shelf of the oven.
Turn out the sponge onto a sheet of baking paper dusted with sugar and peel off the lining paper. Cover with a damp tea towel and leave to cool.
For the filling, soak the gelatine a bowl of cold water for 10 minutes. Finely grate the lemon zest.
Squeeze the juice from the lemon. Heat 2 tbsp lemon juice in a pan, squeeze the water from the gelatine and add to the lemon juice, stirring until dissolved.
Mix the quark with the sugars.
Mix in the remaining lemon juice. Stir 2 tbsp of the mixture into the gelatine and then return it to the main mixture. Whip the cream until stiff and fold in. Leave to cool for 30 minutes.
Spread the cream over the cooled sponge, trimming off any crusty edges. Roll up the roulade tightly and place, seam side down, on a plate. Chill for at least 2 hours. Dust with icing sugar.