For the chicken, you can either grill or boil. You can do this the day before if preferred.
Grilling: Preheat your grill to medium-high heat. Season the chicken breasts with salt and pepper. Grill the chicken for about 6-7 minutes per side, or until the internal temperature reaches 165°F (74°C). Remove from the grill and let cool.
Boiling: Fill a large pot with water with 2 tbsp of salt and bring to a boil. Add the chicken breasts, reduce the heat to a simmer, and cook for about 15-20 minutes or until the chicken is fully cooked and the internal temperature reaches 165°F (74°C). Remove from the water and let cool.
Once the chicken is cool, cut it into 1/2-inch cubes and set aside.
In a large bowl, combine the cubed chicken, whole grain Dijon mustard, honey mustard, crème fraîche, chopped dill, chopped dill pickles, red onion, and celery.
Toss everything together until the chicken and vegetables are evenly coated with the dressing. Adjust mustard, salt, pepper or creme fraiche to taste if you prefer.
Cover the bowl and refrigerate until serving. Keeps for 3-4 days covered in the fridge!