Heat a large pan over medium/high heat. Add the olive oil, onion and a pinch of salt and pepper and cook for about 3 minutes. Add the garlic and cook for 1 min.
Add the zucchini, 1/2 teaspoon salt and 1/4 teaspoon pepper and cook for 5-7 minutes. Add the stock (start with 2.5 cups, you can always add more at the end). Bring to a boil then simmer for 5 minutes.
Turn off heat and add the basil, avocado, juice of 1 lemon, 1/2 teaspoon salt and pinch of pepper.
Transfer soup mixture (in batches) to a high powdered blender and blend until smooth. You can also use an immersion blender and blend right in the pot. If soup is too thick add a little more stock to thin it out. Add salt and pepper to taste and serve topped with avocado and croutons. Soup can be served hot or chilled.