For the Flummeri, soak gelatine in cold water. Mix the soured milk, 2 tbsp. sugar, vanilla sugar and lemon juice. Squeeze the gelatine and dissolve over a low heat. First stir 2-3 tbsp. soured milk into the gelatine, then stir into the remaining soured milk. Cool until the mixture begins to gel.
Whip the cream until stiff and fold into the soured milk. Divide into four portion moulds (each containing approx. 1⁄8 l) and chill for approx. 4 hours.
For the sauce defrost blueberries. Finely puree the blueberries and 1 tbsp. sugar with a hand blender.
Carefully remove flummery from the edge with a sharp knife. Dip the moulds briefly in hot water and turn the flummery over onto four plates. Serve with berry sauce and pistachios.