Place the chicken breast in a pot, cover with water, bring to a boil, then reduce heat and simmer for 18 to 20 minutes until cooked to 165°F / 74°C.
Remove the chicken, let it cool for 5 minutes, then shred or dice into small bite-size pieces.
In a large bowl, whisk together mayonnaise, Dijon mustard, sweet pickle relish, lemon juice, garlic powder, and black pepper until smooth and creamy.
Add the shredded chicken and diced celery to the bowl and toss until evenly coated with the creamy dressing.
Serve immediately on sandwich bread, croissants, or lettuce cups, or refrigerate for up to 3 days.
