In a large mixing bowl, whisk together the Greek yogurt, Dijon mustard, and lemon juice until the mixture is uniform.
Incorporate the salt, black pepper, and chopped parsley into the yogurt base.
Add the diced chicken, chopped celery, diced red onion, and halved grapes to the mixing bowl.
Fold the ingredients together using a spatula until all components are thoroughly coated with the dressing.
Taste the salad and adjust the seasoning with additional salt or pepper if necessary.
Serve immediately on a bed of greens or refrigerate for 30 minutes to allow flavors to fully integrate.