Place chicken breasts in a pot and cover with cold water by 1 inch. Add a pinch of salt. Bring to a gentle boil over medium-high heat.
Reduce heat to low, cover, and simmer for 12-15 minutes until chicken is cooked through (internal temperature reaches 165°F). Remove chicken and let cool slightly.
Shred or dice the chicken into bite-sized pieces. Set aside.
In a large bowl, whisk together mayonnaise, lemon juice, salt, pepper, and garlic powder (if using).
Add the chicken, celery, parsley, and red onion. Stir gently until everything is evenly coated.
Taste and adjust seasoning as needed. Cover and refrigerate for at least 30 minutes before serving to allow flavors to meld.
Serve cold or at room temperature on bread, lettuce wraps, or with crackers.