Select the blueberries, wash and drain. Wash the lemon thoroughly, dab dry and peel the peel as a thin spiral. Squeeze the lemon. Bring the currant juice, blueberries, 75 g sugar, lemon juice and zest to the boil. Stir cornflour and a little cold water until smooth. Stir into the juice and bring to the boil again briefly.
Remove lemon zest and divide the hot groats into 4 glass bowls. Let it cool down. For the vanilla foam, cut the vanilla pod lengthwise and scrape out the pulp. Whip mascarpone, cream, vanilla pulp and remaining sugar with the whisk of the hand mixer until semi-stiff. Just before serving, spread on the groats and serve decorated with lemon balm