Begin by shredding three cups of cooked chicken. If you haven't cooked your chicken yet, poach it in simmering water for about 15-20 minutes until fully cooked, then shred with two forks. You can also use leftover or rotisserie chicken for convenience.
Dice the celery and slice the scallions. Halve the red grapes to release their sweetness and make them easier to eat.
In a large mixing bowl, combine the shredded chicken, diced celery, halved grapes, and sliced scallions. Toss gently to mix.
In a separate small bowl, whisk together the Greek yogurt, Dijon mustard, lemon juice, salt, and black pepper until smooth and creamy.
Pour the dressing over the chicken mixture. Stir until everything is well coated in the creamy dressing.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes before serving to allow the flavors to meld beautifully. Serve on a bed of greens, in a sandwich, or with whole-grain crackers.