To make the vinaigrette, whisk together the mustard, vinegar, olive oil, salt, and pepper in a small bowl. Set aside.
In a small saucepan, boil 1 cup water. Cook the corn until tender, 2 to 3 minutes. Drain and let cool. In a bowl, combine the corn with the red pepper, celery, and lima beans. Toss with half the vinaigrette.
Toss the watercress with the remaining vinaigrette. Divide it among 6 large bowls. Just before serving, spoon some of the vegetable mixture over each salad and top with several pieces of chicken.