Place chicken breasts in a medium pot. Cover with cold water by 1 inch. Add salt, bay leaf, and peppercorns. Bring to a boil over medium-high heat.
Reduce heat to low. Simmer 18 to 20 minutes until the thickest part reads 75°C (165°F) on an instant-read thermometer. Remove from pot.
Rest chicken on a cutting board for 10 minutes. Dice into ½-inch pieces. Do not shred.
Finely dice celery and red onion. Pat celery dry with a paper towel.
In a large bowl, whisk together mayonnaise, Dijon mustard, lemon juice, garlic powder, black pepper, and a pinch of salt. Taste — it should be noticeably tangy.
Add chicken, celery, red onion, and dill. Fold together with a spatula until evenly coated.
Cover and refrigerate for at least 30 minutes. Serve cold.
