Coarsely chop 1 scallion and add it to the bowl of a food processor with the garlic; pulse until garlic is minced. Add the mayonnaise, buttermilk, and salt and pepper, to taste, and pulse until well combined and smooth. Slice the iceberg into 1-inch thick slices, cutting parallel to the core end and transfer to cold salad plates. Drizzle 2 to 3 tablespoons of the dressing over each slice. Thinly slice the remaining scallion, on a bias, and sprinkle over the salads.
Note: This dressing tastes better as it sits; make it ahead and keep it chilled in the refrigerator.