Wash and stone the plums and cut them into slices. Sprinkle with 2-3 tablespoons of sugar. Cover and let stand for about 30 minutes.
Caramelise 50 g sugar in a frying pan until golden brown. Add muesli and stir in with a wooden spoon. Spread the mixture on baking paper and let it set.
Stir starch with 2 tablespoons of fruit juice until smooth. Boil up the rest of the juice. Stir in starch and simmer for about 1 minute. Fold in sugared plums together with the juice and let cool down.
Mix quark with yoghurt, lemon juice and zest, 2-3 tablespoons sugar and vanilla sugar. Serve with the plums. Break the brittle into pieces and sprinkle them on top.