Wash the plums, drain well, cut in half and stone. Cut the plum halves slightly at the top. Mix the quark, 4 tablespoons of milk, oil, 50 g sugar, 1 packet of vanillin sugar and salt with the whisk of the hand mixer.
Mix flour and baking powder and fold in portions. Finally, work into a smooth dough with your hands. Roll out the dough on a lightly floured work surface (approx. 34 cm Ø) and line a greased tart tin (28 cm Ø) with it.
Layer the plums. Roughly chop the walnuts. Melt the remaining sugar and vanillin sugar in a pan. Add butter and bring to the boil. Add remaining milk and stir in. Fold in the walnuts.
Let the mixture cool down a little and spread it over the plums. Bake in the preheated oven (electric: 200 °C/ gas: level 3) for 45-50 minutes. Cover after half of the baking time. Let it cool down on a cake rack.
Serve decorated as desired with a cream stain and lemon balm.