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.
Note: This dressing
tastes better as it sits; make it ahead and keep it chilled in the