Steam broccoli and cauliflower, covered, 3 to 4 minutes or until crisp-tender. Rinse under cold water, and drain well. Set aside.
Combine mayonnaise, buttermilk, onion, blue cheese, salt, and pepper in a small bowl.
Combine broccoli and cauliflower with dressing in a large bowl. Sprinkle with parsley; stir well.