Add the chopped celery, halved grapes, and toasted walnuts to the bowl.
In a small bowl, whisk together the Greek yogurt, mayonnaise, remaining 2 teaspoons lemon juice, honey, salt, and pepper. Adjust taste as needed with more honey or lemon juice.
Cover the bowl and refrigerate the salad for at least 15 minutes before serving to blend the flavors. Sprinkle with fresh parsley if desired.