I grew up blocks away from the Waldorf Astoria hotel in New York City. I remember walking by the heralded building as a kid and wondering if all the guests inside were eating nothing but Waldorf salad. I imagined the hotel must have needed a lot of grapes, walnuts and mayonnaise! When I first made the salad as a young chef in Larry Forgione's kitchen decades later, I marveled at how good this salad can be when it's treated with care. A small cheffy note: The confectioners' sugar on the grapes brings out more of their vibrant color when you roast them, making for a tasty, delicate balancing act between them and the lettuce on one side and the roasted chicken, crunchy walnuts and mayonnaise-style dressing on the other. I like to mix this salad together at the very last minute so the flavors are all still independently delicious and therefore even better when eaten all together.
Toast the walnuts: Place the walnuts on a rimmed quarter sheet pan and toast them in the oven until they are fragrant and browned, 5 to 8 minutes. Set aside to cool.
Roast the grapes: In a medium bowl, toss the grapes with the confectioners' sugar and 1 tablespoon of the olive oil. Arrange the grapes in a single layer on a rimmed sheet pan and bake until their skins start to wrinkle, 12 to 15 minutes. Remove the grapes from the oven and set them aside to cool.
Make the dressing: To a blender, add the lemon zest and juice, vinegar, mustard and granulated sugar. Blend until smooth and, with the blender running, slowly and steadily add the remaining 5 tablespoons olive oil. Taste for seasoning, adding more vinegar or salt if needed. If the flavor is too strong or the dressing is too thick (or separates a little), blend in a splash of cool water.
Assemble the salad: Arrange the lettuce leaves on a serving platter. Toss the apples and celery ribs with a some of the dressing in a large bowl; set aside. Scatter the chicken over the lettuce, followed by the toasted walnuts, and drizzle with some of the dressing. Scatter the roasted grapes and dressed apples and celery over the top. Drizzle with more dressing and scatter the inner celery leaves over the top.