Peel the cucumber, slice in half, length-wise, and using a spoon, scoop out the seeds. Chop into large, rough pieces.
Chop into rough, coarse pieces the celery, shallot and one avocado. Peel the garlic. Place all of the above mentioned ingredients (cucumber, avocado, shallot, celery and garlic) into a food processor.
Before blending, add to the food processor the spinach, olive oil, water, vinegar (champagne or white wine), salt and cayenne pepper. Blend/purée until smooth. Be sure to taste and season with more water or pepper or salt.
Place in a bowl, cover and place in the refrigerator to chill for at least 2 hours. I have made mine a day ahead and it was wonderfully cool and refreshing.
To serve, prepare the three items for topping (dice the avocado, keep small leaves of parsley, etc.). Distribute the soup into bowls and garnish with your preferred toppings. I have also used greek whole-fat yogurt which adds a lovely richness. Enjoy!