Halve and pit two avocados. With a spoon, scoop out flesh and transfer to a blender. Add buttermilk, walnuts, dill, red onion, vinegar, salt, and 1 cup water and puree until smooth.
Cover the blender and refrigerate until the soup is well chilled, at least one hour. Halve and pit remaining avocado. Cut into four sections lengthwise, and then cut crosswise into 1/2-inch chunks. Divide soup among four bowls and garnish each with diced avocado and dill, if desired.
You can slow the oxidation process by just squeezing a little lemon or lime juice on it. Tip: Diced avocado makes a nice garnish to this soup, but you could also easily serve it on its own.