In a large mixing bowl, combine Greek yogurt, Dijon mustard, garlic powder, and a pinch of salt and black pepper. Whisk until smooth and creamy.

Fold in the shredded or chopped cooked chicken breast until evenly coated.

Add sliced pepperoncini, chopped celery, and diced red onion. Mix until well distributed.

Taste and adjust seasonings if needed, adding more salt or pepper. Sprinkle in chopped parsley or dill if desired.

Cover and refrigerate for at least 15-30 minutes to allow flavors to meld.

Serve in crispy lettuce cups, on whole grain bread, or with whole wheat crackers.