In a mixing bowl, combine the mayonnaise, Dijon mustard, lemon juice, garlic powder, and a pinch of salt and pepper. Whisk until smooth and well-combined.
Add the shredded chicken, celery, and red onion to the bowl. Use a fork or spatula to gently fold the ingredients into the dressing until everything is coated evenly.
Give the salad a taste and adjust the seasoning if necessary. Add more salt, pepper, or lemon juice according to your preference.
Cover the salad and let it chill in the refrigerator for at least 30 minutes before serving.
