When shopping for avocados, choose those that are firm to the touch with unbroken skin. Let it ripen a few days at room temperature. You'll know it's ready to eat when it feels soft. Once cut, avocados will turn brown quickly. But 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.