Start with chicken that's already cooked and fully cooled, since warm chicken will make your dressing separate. Shred or dice it into bite-sized pieces, whichever texture you personally prefer in your salad. Rotisserie chicken works wonderfully here if you want to skip a cooking step entirely.
In a small bowl, combine the Greek yogurt, mayonnaise, honey, apple cider vinegar, poppy seeds, salt, and pepper. Whisk everything together until it's completely smooth and well blended. Taste it and adjust the salt or honey slightly if you want it sweeter or saltier.
While your dressing sits, finely chop your celery, red onion, and pecans on a cutting board. Halve your grapes as well, since whole grapes can make each bite a bit awkward to eat. Keeping everything a similar small size helps the salad hold together nicely.
Add your shredded chicken to a large mixing bowl, then pour the dressing over the top. Toss in the celery, grapes, onion, and pecans, folding everything together gently with a spatula. You want every piece of chicken lightly coated without mashing the ingredients.
For the best flavor, cover your bowl and refrigerate the salad for at least 15 minutes before serving. This short rest lets the dressing really soak into the chicken and vegetables. Serve it cold, straight from the fridge, for that classic deli-style taste.