In a dry skillet over medium heat, add the walnuts and toast for 2 to 3 minutes, tossing every 30 seconds. Remove from heat and allow to cool before chopping and adding to salad.
In a large mixing bowl, combine ⅓ cup plain Greek yogurt, ⅓ cup mayonnaise, the juice of 1 small lemon, ½ teaspoon kosher salt, and ¼ teaspoon pepper. Whisk until smooth and creamy.
Add the chicken, halved grapes, diced apples, minced celery, and chopped toasted walnuts to the dressing and toss until everything is evenly combined and coated in the dressing.
Cover and refrigerate for at least 30 minutes before serving.
Serve this chicken salad on a bed of lettuce, on a wrap, on crackers, or as a sandwich.
