In a medium pot, combine the chicken stock and water and bring to a boil.
Add the chicken breast to the boiling liquid, reduce the heat to maintain a gentle simmer, and cook until no longer pink inside, about 10–12 minutes.
Drain the chicken and let it cool slightly, then shred or chop into bite-sized pieces.
In a mixing bowl, whisk together the Whole30 mayonnaise, Dijon mustard, fresh lemon juice, salt, and dried dill until smooth.
Add the sliced celery to the dressing and stir to combine.
Fold the shredded chicken and sliced grapes into the dressed celery mixture until everything is evenly coated.
Serve immediately or chill briefly before serving.
