In a medium bowl, prepare the dressing by whisking together the mayonnaise, Greek yogurt, and Dijon mustard until smooth and fully incorporated.
Add the shredded chicken, chopped spinach, diced celery, diced red onion, and fresh dill to the bowl with the dressing.
Gently stir all the ingredients together until the chicken and vegetables are evenly coated. Season with salt and pepper, and mix once more to combine.
Serve immediately for the best texture, or chill for up to 30 minutes to allow the flavors to meld.