Preheat a small skillet over medium heat. Add walnuts and toast, stirring frequently, until fragrant and lightly browned, about 4–6 minutes. Transfer to a plate to cool.

Wash apples and grapes. Core apples and dice into roughly 1/2-inch (1–1.5 cm) cubes. Immediately toss diced apples with the lemon juice to prevent browning.

Thinly slice celery on a bias (about 1/8–1/4 inch thick). Halve grapes. If using cheese, crumble or slice thinly now.

Dressing

In a medium bowl, combine the Greek yogurt and mayonnaise (if using). Whisk until smooth.

Add honey, lemon zest, lemon juice, and Dijon mustard (if using). Whisk to combine.

Season with salt and freshly ground black pepper. Taste and adjust sweetness or acidity: add more honey for sweetness or a little more lemon juice for brightness.

Assemble

In a large mixing bowl, combine the prepared apples, celery, and halved grapes.

Pour the dressing over the fruit and celery. Gently fold with a spatula until all pieces are evenly coated. Start with about three-quarters of the dressing and add more if you prefer a creamier salad.

Fold in the toasted walnuts, reserving a tablespoon for garnish. If using cheese, gently fold it in now or sprinkle on top when serving.

If serving on a bed of greens, arrange the greens on a platter or individual plates and spoon the Waldorf Salad on top.

Garnish with the reserved toasted walnuts and an extra grind of black pepper. Serve immediately or chill for up to 1 hour to meld flavors. If chilling longer than 1 hour, stir again before serving and add any additional dressing as needed.