Onions peel and cut into slices. Mix vinegar and broth and bring to the boil. Blanch the onions in it for about 3 minutes, pour off, catch the stock and let both cool down.
In the meantime wash, clean and halve the cucumber. Cut the halves lengthwise into thin slices. Wash and clean the mushrooms briefly and cut the larger mushrooms in half. Clean, wash and pluck lettuce into bite-sized pieces.
Cut the cheese roll into slices. Wash the chives and cut into fine rolls. Mix vinegar stock, some salt and pepper and add oil. Add chives and season the vinaigrette if necessary.
Arrange prepared ingredients on a plate, pour vinaigrette over it and serve.