Wash the lemon hot and grate half of the peel thinly. Halve the lemon and squeeze the juice. Separate the eggs. Beat the egg whites until stiff, adding salt and 125 g sugar. Stir in egg yolks and lemon peel.
Mix flour and baking powder, sieve over the egg mixture and carefully fold in. Pour the sponge mixture into a greased springform pan (approx. 26 cm Ø) and smooth it down. In a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see
manufacturer) bake for about 25 minutes. Take the cake out of the oven, remove from the edge and let it cool down.
Defrost the blueberries. Soak gelatine in cold water. Cut the vanilla pod lengthwise and scrape out the pulp with the back of a knife. Stir mascarpone, 100 g sugar, vanilla pulp, 3 tbsp lemon juice and quark until smooth.
Dissolve the squeezed-out gelatine in a small pot. Stir in 3 tablespoons of cream, then stir into the remaining cream. Whip cream until stiff and fold in.
Cut the cake base in half horizontally. Place a cake ring around the lower cake base. Spread 2/3 of the cream evenly on the cake base. Place the sponge cake lid on top and spread the rest of the cream loosely.
Chill the cake for about 2 hours.
Mix starch and apple juice. Bring blueberries to the boil in a pot, stir in the mixed starch and simmer for about 1 minute, stirring continuously. Remove from the heat and let it cool down. Remove the cake from the ring.
Spread the blueberries on the cake, chill again for about 2 hours.