If the walnuts are not already toasted, toast them in a dry skillet over medium heat for 3 to 4 minutes, stirring often, until fragrant; cool, then roughly chop.
In a large bowl, whisk the Greek yogurt, olive oil, lemon juice, Dijon, salt and pepper until smooth.
Add the chicken, grapes, celery, green onions and walnuts; fold gently until everything is evenly coated.
Cover and refrigerate for at least 20 minutes so the flavors meld.
Serve on whole-grain bread, in a wrap, or over lettuce; store covered in the fridge up to 4 days.