Wash the potatoes and boil in boiling water for about 25 minutes until soft. Peel and finely dice the onion. Boil up onion cubes, broth, vinegar, mustard and oil, season with salt and pepper. Let simmer for about 3 minutes. Drain the potatoes, rinse briefly under cold water and peel.
Cut the larger potatoes in half. Pour hot marinade over the potatoes and leave to stand for at least 1 hour. Roast the nuts in a pan without fat. Take them out, let them cool down briefly and chop them coarsely. Wash and halve the grapes. Wash parsley, shake dry and chop leaves coarsely except for a little bit for garnishing.
Cut the cheese into cubes. Clean, wash and roughly pluck the radicchio into pieces. Add nuts, grapes, chopped parsley, cheese and radicchio to the salad before serving, then season to taste again. Garnish with parsley