Place the zucchini, 1/2 cup water, tomato, celery, green onion, lemon juice, miso, garlic, cayenne, and salt in a blender and process until smooth. Add the spinach and basil and blend again. Add the avocado and blend until smooth. Add the remaining 1/4 cup water to thin, if necessary, and blend briefly. Serve immediately. For a chilled soup, refrigerate for 2 hours before serving.