Chicken Salad Sandwich

Name: Chicken Salad Sandwich
Ingredients: 1 teaspoon salt; 1/2 teaspoon freshly ground black pepper; 1 pound boneless, skinless chicken breast (2 small whole breasts); 3/4 cup plain nonfat yogurt; 1 tablespoon Dijon mustard; 2 tablespoons snipped fresh chives; 1 tablespoon chopped fresh tarragon; 1 Granny Smith apple; Juice of 1/2 lemon; 1 cup finely diced fennel; 1/2 cup finely diced celery; 2 cups halved red seedless grapes; 6 slices pumpernickel bread; 1 bunch watercress, tough stems removed; Olive-oil, cooking spray