Chill salad plates when you begin preparing your entree.
When your entree is almost ready to serve, combine arugula and romaine leaves and separate onto 4 chilled salad plates. Scatter toasted walnuts evenly among the salads. Pour vinegar into a small bowl and whisk in extra-virgin olive oil in a slow stream to combine dressing. Season dressing with salt and pepper, then stir in blue cheese crumbles. Ladle dressing evenly over top of salad plates and serve.