Place the chicken breasts in a large pot and cover with cold water by 1 inch. Add the bay leaf, peppercorns, and salt. Bring to a gentle simmer over medium heat and cook 12–15 minutes, until the internal temperature reaches 165°F.
Remove the chicken and let it rest on a cutting board for 10–15 minutes, until just warm. Shred with two forks or dice into small cubes.
While the chicken rests, finely dice the celery and red onion.
In a large mixing bowl, whisk together the mayonnaise, lemon juice, Dijon mustard, chopped tarragon, salt, and pepper until smooth.
Add the shredded chicken, celery, and onion to the dressing. Fold gently until evenly coated. Fold in the grapes last, if using.
Cover and refrigerate for at least 1 hour before serving.