Remove the outermost layer of the fennel and cut off the thick green stalks. Cut off the tender greens and set aside. Wash the fennel, cut in half and cut into fine half rings.
Peel the oranges with the knife so that the white skin is also removed. Cut the fruit into thin slices, collecting the juice. Cut the onions into wafer-thin rings.
Arrange the oranges, onions and fennel on a plate or in a shallow bowl. Wash the rosemary, spin dry and pluck the needles off.
Chop the fennel greens and sprinkle with the rosemary over the salad.
Mix the collected orange juice with oil, vinegar, salt and pepper thoroughly, pour over the salad and let it steep briefly before serving.