Prepare the chicken by shredding or dicing it into bite-sized pieces. If using raw chicken, cook it until fully cooked and let it cool before shredding.
Chop the celery into small pieces and halve the red grapes. If using green onions, slice them thinly.
In a large mixing bowl, combine mayonnaise, Greek yogurt, Dijon mustard, lemon juice, garlic powder, onion powder, salt, and black pepper. Mix until smooth and creamy.
Add the shredded chicken, diced celery, halved grapes, and any optional ingredients like walnuts and green onions to the bowl with the dressing. Gently stir until well coated.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes before serving.