Dice butter for the bottom. Knead flour, 50 g sugar and butter cubes to a smooth dough. Roll out dough between 2 layers of foil to a circle (approx. 25 cm Ø). Line a springform pan (24 cm Ø) with it, press the rim slightly. Place the mould in a cool place for about 30 minutes.
Separate the eggs for the filling. Mix quark, cream cheese, egg yolks, custard powder and lemon peel. Beat the egg whites until stiff, then pour in 100 g sugar. Fold into the quark mixture. Pour into the springform pan and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for about 50 minutes. Remove from the oven, carefully remove the rim from the tin, let it cool down well on a cake rack in the tin
Drain the blueberries and collect the juice. Place the blueberries in a bowl. Stir 4 tablespoons of juice with starch until smooth, put the remaining juice and lemon juice in a pot and bring to the boil. Stir in the starch and let it simmer at medium heat for about 1 minute. Remove from heat and pour over the blueberries. spread the blueberries on the cake, let it cool down
Preparation time approx. 1 3/4 hours. waiting time approx. 2 hours