Pit the avocadoes and place their flesh in a food processor, with onion, garlic, cilantro, mint and half of the stock. Process until smooth. Add the rest of stock, rice vinegar, and soy sauce. Process again until smooth. Season with salt and pepper.
Chill at least 15 minutes before serving.
You can garnish this soup with sour cream that has been blended with cilantro and lime juice.