Rinse and trim fennel; then separate it into layers as you would celery ribs. 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 lemon juice.
Peel 3 oranges, removing as much of the white pith as you can. Cut peeled oranges crosswise into 1/8-inch-thick disks and set aside.
Juice the remaining orange; then whisk juice together with remaining tablespoon lemon juice plus mustard, salt, and cayenne. Slowly drizzle in olive oil, whisking as you go.
Arrange 3 or 4 slices of orange on each of 6 salad plates. Drain fennel strips and scatter them over a clean kitchen towel. Pat dry and arrange fennel over the orange slices.
Scatter olives over fennel strips and place a few sprigs of watercress or m