How to Make It
Heat oven to 400° F. Rub the chicken breasts with oil. Place on a rimmed baking sheet and roast until cooked through, 14 to 16 minutes; let cool, then shred.
In a medium bowl, mix together the mayonnaise, tarragon, and lemon zest and juice. Fold in the chicken, scallions, and celery; season with ¾ teaspoon salt and ½ teaspoon pepper. Serve over the watercress.