In a medium mixing bowl, add the Greek yogurt (or 50/50 yogurt mayo combo), Dijon mustard, honey, lemon juice, rosemary, salt, and pepper. Stir to combine.
Add the shredded chicken, celery, grapes, and green onion. Fold to combine.
Refrigerate for 30 minutes or longer for the flavors to meld together.
Serve on toasted bread, top with arugula and enjoy!