For the filling, defrost the blueberries in a sieve, collecting the juice. Mix the juice with 1 tablespoon of sugar and starch. Simmer in a pot while stirring for 1-2 minutes. Stir in the berries and let them cool down.
Preheat oven (electric cooker: 225 °C/circulating air: 200 °C/gas: see manufacturer). Line a baking tray with baking paper. Separate the eggs for the sponge cake. Beat the egg whites with 1 pinch of salt until stiff, adding 50 g sugar.
Stir in the egg yolk. Sift the flour on top and fold in. Spread the sponge mixture on the baking tray to form a rectangle (approx. 20 x 40 cm). Bake in a hot oven for about 8 minutes. Moisten a kitchen towel slightly and sprinkle with 2 tbsp. sugar.
Turn the sponge cake onto the kitchen towel. Carefully remove the paper. Roll up the sponge cake from the long side using the cloth. Let it cool down.
Mix quark, 2 tablespoons sugar and cream stiffener. Stir in 1 tablespoon of thickened berries. Unroll the sponge cake and cut into 8 strips (approx. 5 x 20 cm each). Spread a thin layer of quark cream on each strip and spread about 2 tsp. berries on each strip.
Carefully roll up the strips, stand upright. Spread the rest of the quark cream all around.