Soak the cubed bread in a little water until it has softened. Squeeze out the excess water and set aside.
Slice the cucumber in half lengthwise and scrape out the seeds using a small spoon. Discard the seeds and roughly chop the cucumber. Deseed and roughly chop the pepper. Roughly chop the tomatoes. Place these vegetables in a large stand blender. Add the softened bread, avocado, green garlic, coriander (including the stems), 1/2 cup greek yogurt, olive oil, vinegar, salt and pepper. Pour in one cup of water and blitz until you have a smooth consistency. Taste the soup and adjust the salt and pepper to your liking.
Using a medium mesh sieve, strain the soup into a large bowl. You may need to use a spatula to help the liquid pass through the sieve. Be sure not to use a fine sieve or you won’t have any soup! Cover the bowl with plastic wrap and chill for at least 3 hours. Serve chilled with some greek yogurt and a sprinkle of cayenne powder if desired.
If you cannot find green (baby) garlic, you can add one clove of garlic instead.