Mix the flour and salt in a bowl. Add the quark, water and vinegar, and mix quickly to create a soft dough – do not knead. Divide the dough into 4 portions, on a surface lightly dusted with flour shape into rolls each approx. 18 cm diameter and transfer to two oven trays lined with baking paper.
Sprinkle the nuts on the middle of the dough rounds. Spread the plums on top, leaving a border of approx. 4 cm. Distribute butter and sugar on them. Turn the edge of the dough in on the plums.
Approx. 20 mins. in an oven preheated to 200 °C (convection). Remove from the oven, transfer to a cooling rack with the baking paper and leave to cool.
Dust the tarts with icing sugar.