Crumble the yeast and dissolve in 800 ml of lukewarm water. Knead in a large bowl with all the flour, sourdough, salt, sugar and caraway seeds to a smooth dough. Cover the dough and leave to rise in a warm place for about 1 hour until it has doubled in volume.
Knead the dough briefly on a little flour. Divide into about 16 pieces and form into round flat cakes. Place on three baking trays lined with baking paper and dust with flour. Cover and leave to rise for another 10 minutes. Bake one after the other in a preheated oven (electric oven: 225 °C/circulating air: 200 °C/gas: see manufacturer) for about 25 minutes. Let them cool down.