Mix the strawberries, sugar, lemon zest and juice, cover and set aside.
Mix together the quark, sugar and vanilla sugar in a bowl. Dissolve the gelatine in the water, combine with 2 tbsp of the quark mixture and immediately stir into the remaining mixture. Whip the cream until stiff, place on top of the quark mixture.
Beat the egg whites with the salt until stiff, carefully fold into the quark mixture along with the whipped cream.
Brush the inside of a springform pan (approx. 24 cm) with a little oil and line with cling film. Place the closed pan on a cake plate. Cover the base with tightly packed sponge fingers (sugar side down). Carefully spoon the reserved strawberries and resulting juice on top of the sponge fingers. Top with the quark mixture, then cover and leave to set in the fridge for approx. 4 hrs.
Carefully remove the tin frame. Decorate the cake with strawberries.