Place the eggs, sugar, vanilla sugar, salt and finely grated lemon zest in a mixing bowl. Whisk with the whisk of the hand mixer in a hot water bath until thick and creamy. Remove the bowl.
Mix flour and starch, sieve over the egg cream and carefully fold in with a whisk. Half grease the baking tray and line with baking paper. Spread the sponge cake mixture on the prepared baking tray.
Pinch a bar of aluminium foil and place it in front of the dough so that it does not run apart. Bake in the preheated oven (electric oven: 175 °C/ gas: level 2) for approx. 10-12 minutes. Turn out the dough sheet onto a tea towel sprinkled with sugar.
Carefully remove the baking parchment immediately and turn the pastry sheet with the tea towel into a roll. Let it cool down on a cake rack. For the filling, soak the gelatine in cold water.
Whip the cream until stiff. Grate the lemon, squeeze the juice. Mix juice and peel with quark, vanillin sugar and sugar. Squeeze the gelatine, dissolve carefully and stir into the quark mixture. Fold in cream and blueberries, except for a few for garnishing.
Use frozen fruit unfrozen. Unroll the sponge roll again. Spread the quark and fruit cream on top and roll up again. Roll out marzipan between 2 layers of cling film approx. 2 mm thick and cut out 3 small flowers.
Whip the rest of the cream until stiff. Fill 1/3 of it into a piping bag with star spout. Spread the roll with the rest of the cream. Decorate with a jagged pastry sheet and garnish with the slightly compressed marzipan flowers filled with blueberries.
Put cream dabs in between and sprinkle with ground pistachios.