In a medium bowl, combine diced celery, diced red onion or sliced scallions, quartered grapes, and chopped fresh dill.
Add the shredded chicken to the bowl with the vegetables and fruits.
Add mayonnaise (or yogurt-mayo combination), Dijon mustard, fresh lemon juice, and celery seeds if using.
Stir until all ingredients are evenly combined and coated with dressing.
Season with kosher salt and freshly ground black pepper to taste.
If using pecans, fold them in gently just before serving.
For best flavor, refrigerate for at least 30 minutes before serving to allow flavors to meld.