For the Dressing, mix mayonnaise, buttermilk and sour cream in medium bowl. Stir in vinegar, garlic, smoked black pepper and onion salt until well blended. Cover. Refrigerate at least 1 hour to blend flavors.
For the Spinach Salad, toss spinach, pears, red onion and walnuts in large bowl. Divide among salad plates. Sprinkle with blue cheese. Serve with Dressing.