Place chicken breasts in a medium saucepan. Add cold water, salt, bay leaf, peppercorns, and halved garlic. Cover chicken by at least 2.5cm of water.
Bring to a gentle simmer over medium heat. Reduce to medium-low once bubbles appear. Do not boil.
Simmer 12–15 minutes until chicken reaches an internal temperature of 74°C (165°F) and is completely white at the thickest point.
Transfer chicken to a cutting board and rest 10 minutes. Pull apart into 1.5–2cm chunks with two forks.
Soak diced red onion in cold water for 5 minutes. Drain and pat dry.
Whisk together mayonnaise, Dijon mustard, lemon juice, salt, pepper, and garlic powder in a large bowl.
Add cooled pulled chicken, celery, drained red onion, and fresh dill. Fold gently with a rubber spatula.
Taste and adjust seasoning. Cover and refrigerate at least 20 minutes before serving.