Toast nuts in a small skillet over moderate heat in a small pan until aroma develops and nuts crisp a bit, 5 to 6 minutes. Set aside to cool.
Combine walnuts, celery, onion, grapes and apples in a medium bowl. Whisk lemon juice, honey, vinegar, cardamom, dill and mustead, then whisk in extra-virgin olive oil in a slow stream. When oil becomes incorporated stream in cream, then season the dressing with salt and pepper, to taste.
Dress salad and adjust salt and pepper, to taste. Use 2 large lettuce leaves per serving to form a bed for the salad.