**Prepare the Dressing:** In a small bowl, whisk together mayonnaise, Greek yogurt, lemon juice, honey (if using), salt, and pepper until smooth. Adjust seasoning to taste.
**Combine Ingredients:** In a large mixing bowl, combine the chopped apples, grapes, celery, and walnuts.
**Toss the Salad:** Pour the dressing over the salad ingredients and toss gently until everything is evenly coated.
**Chill:** Refrigerate for at least 15 minutes to allow the flavors to meld.
**Serve:** Serve chilled, optionally on a bed of lettuce leaves or garnished with parsley for a pop of color.