For the Dressing:
This is a modern interpretation of the classic Waldorf salad–a creation of Oscar Tschirky, the first maitre d'hotel of the Waldorf Astoria in New York. The original Waldorf salad was just apples and celery in mayonnaise. Over time, raisins and walnuts were added. But we've never been fans of fruit in mayonnaise or raisins in salads. So we replaced the raisins with red grapes and tossed them with apples and lettuce in a simple lemon vinaigrette. Then we mixed chicken, celery, and lots of toasted walnuts in a creamy dressing of Greek yogurt with mayo, lemon juice, tarragon, and onion. It's like a chicken salad mixed with a green salad combined with a fruit salad. And all those parts come together in a fresher, more flavorful whole.
Note: A 1-½-pound rotisserie chicken will give you enough meat for this salad. You can substitute fresh chives, chervil, or mint for the tarragon.
In a small bowl, whisk together 1 tablespoon lemon juice with the olive oil and ¼ teaspoon salt. Set aside. In another small bowl, stir together the remaining 3 tablespoons lemon juice, yogurt, mayonnaise, tarragon, honey, lemon zest, remaining ¼ teaspoon salt, and pepper. Stir in onion. Set aside while you prepare the salad.
For the Salad:
Toast walnuts in a large dry skillet over medium-low heat, stirring occasionally, until brown and fragrant, about 10 minutes. Pour into a medium bowl, and let cool as you prepare the chicken: Remove any skin and tear the chicken into 2- or 3-inch strips. Add to the bowl with the walnuts. Add the celery and the yogurt dressing and stir so that everything is evenly covered.
In your serving bowl, toss lettuce, grapes, and apple slices with lemon/oil dressing. Spoon chicken mixture over all. Use your hands to fluff leaves and grapes lightly, just to make it look pretty. Serve on chilled salad plates.