Rinse and trim the fennel and then separate into layers. Slice each layer into 2-inch strips and then transfer to a bowl. Add enough cold water to cover by 1-inch. Stir in 1-tablespoon of the lemon juice. Peel 3 oranges, slice crosswise and set aside. In a small bowl whisk together the remaining lemon juice, the juice of the remaining orange and the mustard. Blend in the salt and cayenne, then whisk in the oil. (Or combine all dressing ingredients in a jar with a tight-fitting lid and shake well.) Arrange 3 or 4 slices of orange on each of 6 plates. Drain the fennel strips, pat dry and arrange over the orange slices. Scatter olives over all and arrange a few sprigs of watercress on the side. Pour on the dressing and chill briefly before serving.