Combine salt and pepper in a bowl. Heat a large grill pan over medium-high heat. Coat with cooking spray. Sprinkle chicken with some of the salt mixture; place in pan. Reduce heat to medium; cover. Cook until chicken is cooked through, about 12 minutes (flip halfway through cooking time). Remove from pan; set aside. Note: can also grill chicken on an outdoor grill, same procedure applies.

In a bowl, combine yogurt, mustard, chives, tarragon, and remaining salt mixture. Core the apple, and cut into 1/4-inch dice. Place in a medium bowl with lemon juice, and toss to combine. Add fennel, celery, and grapes. Cut reserved chicken into 1/2-inch pieces. Add to salad with yogurt dressing; stir to combine.

Serve on toasted pumpernickel bread, open-faced, with watercress.