Drain the canned chicken thoroughly using a fine-mesh strainer. Press gently with a fork to remove excess liquid.
Transfer the drained chicken to a large mixing bowl and use two forks to gently shred it to your desired consistency.
In a separate small bowl, whisk together the mayonnaise and Dijon mustard until smooth. Add pickle relish, salt, pepper, and garlic powder (if using) and mix until well combined.
Add the diced celery and red onion to the bowl with the shredded chicken.
Pour the prepared dressing over the chicken mixture and gently fold everything together using a rubber spatula until evenly coated.
If using optional add-ins like diced apple, dried cranberries, or chopped nuts, fold them in now.
Stir in the fresh parsley and lemon juice (if using) as the final touch.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes before serving to allow flavors to meld.
