Crumble 1 cup of the blue cheese and set it aside as a garnish.
Set up a food processor and, with the motor running, drop the scallions through the feed tube. After 10 seconds, shut off the machine and scrape down the sides of the bowl. Add the remaining chopped blue cheese and pulse a few times to chop it further. Stop the motor and add the mayonnaise, vinegar, and 2 tsp. black pepper. With the motor running, add the buttermilk. Check the texture; if it is too thick, add more buttermilk to produce a thick and creamy, but pourable, dressing. (The dressing may be prepared up to 3 days ahead; store in an airtight container in the refrigerator.)
To assemble the salad, lay the iceberg wedges cut side up on 8 salad plates. Spoon about 1/4 cup of the dressing over each, then sprinkle each with some of the reserved blue cheese and a few grinds of black pepper. Leftover dressing will keep for 1 week in an airtight container in the refrigerator.
nutrition information (per serving):
Photo: Christopher Hirsheimer; food styling by Melissa Hamilton