Fill a large saucepan with water and bring to the boil. Using a slotted spoon, gently lower the eggs into the water. Cover and simmer for 10 minutes. Drain the eggs and run under cold water until completely cooled. Peel the eggs and chop them.
Cook the bacon in a large skillet over medium heat, turning occasionally, until crisp, 10 to 12 minutes. Remove to a paper towel-lined plate to drain, then roughly chop.
Whisk the mayonnaise, buttermilk, sour cream and Worcestershire sauce in a large bowl. Stir in the blue cheese and chives, reserving some of the chives for garnish, then season with salt and pepper. Add the chicken, tomatoes, scallions, avocado, bacon, hard-boiled eggs and parsley, reserving some bacon, egg and parsley for the salad platter garnish, and toss to coat completely. Taste and add more salt and pepper, if needed. Refrigerate for at least 30 minutes and up to overnight.
To serve as a salad: Lay the chicken salad on a bed of greens. Garnish with some of the egg, bacon, chives and parsley.
To serve as a sandwich: Lay the chicken salad between slices of sourdough bread.