Bring vinegar, sugar, olive oil and chilli powder to the boil. Season with salt and pour over the dried plums. Set aside.
For the yeast dough mix flour and salt. Make a well in the middle and add the yeast and sugar dissolved in 125 ml water. Mix with some flour. Cover and leave to rise in a warm place for about 10 minutes.
Add olive oil and knead everything with dough hooks to a smooth dough. Cover and let it rise for about 30-45 minutes.
Peel onions and cut into thin rings. Heat 1 tablespoon of olive oil in a coated pan and fry the onions until transparent. Add rosemary, fry briefly. Season with little salt and plenty of pepper.
Knead the yeast dough, divide into 6 portions. Roll out into flat cakes on a lightly floured work surface. Place on baking trays covered with baking paper. Prick several times with a fork and brush with the remaining olive oil.
Spread the onions on top. Chop the walnuts and sprinkle over them. Cover with cheese slices. Bake in the preheated oven at 200-220 degrees (gas: level 4-5, fan oven: 180-200 degrees) for about 15-20 minutes.
For the dried plums, clean and rinse the spring onions and cut them into fine rings. Heat the dried plums lukewarm in the stock. Mix in the spring onions. Rinse radicchio leaves, dab dry and cut into strips.
Place on the onion focaccia breads and spread marinated dried plums on top. Serve immediately.