Peel the oranges with a sharp knife, removing all the white skin. Then cut out segments, cutting inside the separating skins. Cut the segments in half if desired. Catch any juice.
Wash and trim the fennel and cut in half lengthwise. Cut out the stalk and dice finely. Reserve a few of the fennel leaves. Peel the onion and cut into rings.
Put the orange pieces into a bowl with the fennel and onion rings. Mix the olive oil with the orange juice, rosemary and a little pepper and mix with the salad.
Serve in bowls sprinkled with a few fennel leaves.