Separate the eggs for the sponge cake. Beat the egg whites until stiff, pour in 180 g sugar. Beat the egg yolks separately. Sift flour, starch and baking powder on top and fold in. Spread into a springform pan (28 cm Ø) lined at the bottom with baking paper. Bake in a preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer) for about 30 minutes. Let them cool down.
For the cream, soak gelatine cold. Cut the sponge in half horizontally. Mix quark, yoghurt, lemon juice and 2 tablespoons of sugar. Squeeze the gelatine and dissolve at low heat. Stir 3 tbsp. yoghurt mass into the gelatine, then stir everything into the rest of the yoghurt. Cool until the mass begins to gel. Whip cream until stiff, fold in.
Place a cake ring around the lower cake base. 3⁄4 Spread cream on it. Place the 2nd cake layer on top. Spread the rest of the cream on top. Chill for at least 5 hours.
Wash blueberries. Cut the lemon first into slices and then into wedges. Remove cake from the ring. Decorate with blueberries, lemon and possibly thyme blossoms.