In a large mixing bowl, whisk together the plain Greek yogurt, Dijon mustard, garlic powder, salt, and black pepper until smooth.

Gently fold in the shredded cooked chicken breast, sliced pepperoncini, chopped celery, and diced red onion into the yogurt mixture.

Taste your salad mix and adjust the seasoning as necessary.

Transfer the salad into an airtight container and refrigerate for 15 to 30 minutes.

Serve the salad on whole grain bread, in lettuce cups, or over a bed of mixed greens.