Strips of fennel and slices of orange are dressed with a citrusy mustard sauce.
1. Rinse and trim the fennel, and then separate into layers as you would ribs of celery. Remove the strings from the outer layers if necessary. Slice each layer lengthwise into 2-inch julienne strips. Transfer to a mixing bowl. Add enough cold water to cover by 1 inch. Stir in 1 tablespoon of the lemon juice.
2. Peel 3 of the oranges. Slice them across and set aside.
3. In a small bowl, whisk together the remaining tablespoon of lemon juice, the juice of the remaining orange, and the mustard. Blend in the salt and cayenne, then whisk in the oil.
4. Arrange 3 or 4 slices of orange on each of 6 salad plates. Drain the fennel strips and scatter them over a clean kitchen towel. Pat dry and arrange the fennel over the orange slices. Scatter the olives over the fennel strips and place a few sprigs of watercress or mâche to one side. Pour on the dressing and chill briefly to serve.