Mix first 4 ingredients in medium bowl. Add blue cheese and stir until well blended. If too thick, thin with buttermilk by tablespoonfuls to desired consistency. (Can be made 1 day ahead. Cover and chill.) Cook bacon in large skillet over medium heat until golden brown and beginning to crisp. Arrange lettuce on plates. Spoon dressing over. Using slotted spoon, transfer warm bacon from skillet onto salads, dividing equally. Garnish with red onion.