Place the chicken breast in a pot and cover with water and a generous pinch of salt. Bring to simmer, cover, and cook for 15-20 minutes until cooked through. Remove the chicken from water and let rest until cool enough to handle. Use a stand mixer with the paddle attachment or two forks to shred the chicken. Let cool completely before tossing with the other ingredients.
Whisk together all of the dressing ingredients in a large bowl. Taste and season with salt as needed.
In a large bowl toss the shredded chicken with the grapes, celery, onion, toasted pecans, and dressing. Serve immediately or store in the refrigerator. This recipe is even better the next day!