Note that the pickled onion must be made at least one week in advance.
Whisk together buttermilk, mayonnaise, yogurt, and lemon juice in a bowl. Fold in blue cheese. Season with salt and pepper. Cover; refrigerate dressing until cold, about 30 minutes (up to 3 days).
Arrange romaine hearts on a platter. Drizzle with 1 1/4 cups dressing. Top with pickled onion, and season with pepper. Serve with remaining dressing on side.