Dice the cooked chicken, halve the grapes, and dice the apples.
Thinly slice the celery and chop the walnuts.
In a small bowl, whisk together the mayonnaise, Greek yogurt, lemon juice, salt, and pepper until smooth.
In a large mixing bowl, combine the chicken, grapes, apples, celery, and walnuts.
Pour the dressing over the salad ingredients and toss until evenly coated.
Chill the salad for 10-15 minutes if desired. Serve on its own, over lettuce, or with a slice of crusty bread.