Combine white and red cabbage and green onions in a medium bowl. In the bowl of a food processor or blender, combine the mayonnaise, buttermilk, garlic, 1/4 cup of the blue cheese, a pinch of salt and freshly ground black pepper. Process dressing until smooth. Pour dressing over cabbage mixture and sprinkle with remaining blue cheese, tossing to coat. Adjust seasonings with salt and pepper, to taste. Cover and chill for 3 to 4 hours.
Yield: about 4 cups