In a small mixing bowl, combine buttermilk, sour cream, blue cheese, onion, lime juice, and cayenne. Whisk to blend well. Cover and refrigerate for at least 1 hour before using. Dressing can be made up to one day in advance.
Place Romaine in bowl and toss with 1/4 cup of the blue cheese mixture. Add salt and pepper to taste. Arrange turkey, bacon, avocado, tomatoes and eggs in rows across Romaine. Drizzle additional dressing over all.