Mix quark, milk, oil, sugar and salt. Mix flour and baking powder and stir about half into the quark mixture. Knead in the rest of the flour. Wrap the dough in foil and chill for about 1 hour
Clean, wash, halve and stone the plums. Roll out the dough to a rectangle (approx. 32 x 39 cm) on a floured work surface and place on a greased baking tray (32 x 39 cm). Put the plum halves back together and place them close together on the dough. Whisk the egg white, spread the plums with it and sprinkle with sugar crystals. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) on the second rack from below for 40-50 minutes
Waiting time approx. 30 minutes