Drain the beans and put them together in a bowl. Cut the two small onions into cubes and add to the beans. Add the salad mayonnaise to the beans and onions and mix everything well.
Remove the stalks from the dill and chop them into small pieces. Season the salad with salt and pepper, add the dill and mix well again.
Cover with a plate or cling film and put in the refrigerator overnight so that the salad can steep. Decorate with parsley.