In a small bowl, whisk the vinegar, water, mustard, dill, nutmeg, salt and pepper. Slowly add oil while whisking. In a large bowl, combine the apples, blue cheese, walnuts and 3 tablespoons dressing; toss to coat. Cover and refrigerate for up to 4 hours. Cover and refrigerate remaining dressing.
Just before serving, combine romaine and watercress in a large salad bowl; drizzle with remaining dressing and toss to coat. Divide among 12 salad plates; top with apple mixture.