In the jar of a blender, combine the tomatoes, juice, garlic, onion, oregano, vinegar, lemon zest, and some salt and pepper. Puree the mixture until smooth and adjust the seasonings as needed with some salt and pepper or a dash more vinegar. Chill the soup for at least 1 hour or until ready to serve.
To serve the soup, toss the chopped avocados and feta together with the juice of the lemon. Ladle the soup into chilled bowls and garnish with the avocado & cheese. Serve.