In a bowl, toss apples with the cider vinegar. Fold in mayonnaise thoroughly. Season with salt and pepper, to taste. Fold in walnuts, raisins, curry powder, celery, mint, and red onion. Adjust seasoning as desired. Refrigerate for at least 1 hour to allow flavors to incorporate. To serve, arrange 2 leaves of romaine lettuce per plate and spoon salad on top.