For the blue cheese dressing: In a bowl, whisk together the mayonnaise, buttermilk, sour cream and Worcestershire until smooth. Stir in the blue cheese crumbles, chives, salt and pepper. Taste and add more seasoning if needed. Cover and refrigerate for at least 2 hours before serving; the flavors deepen as it chills!
For the chicken: Heat a grill pan or heavy skillet over medium-high heat. Drizzle on a small amount of olive oil.
Salt and pepper the chicken, then grill until totally done, 2 to 3 minutes per side. Remove from the pan and set aside to cool.
For the salad: Arrange the Bibb, romaine and iceberg lettuce in a large bowl or on a large platter in 3 separate sections. Arrange the bacon, tomatoes, eggs and avocados in different piles. Pile the blue cheese crumbles in the center, scatter over the scallions and sprinkle the whole salad with a little black pepper.
Drizzle the blue cheese dressing all over the salad and serve.