Prepare an ice water bath in a large bowl before you begin cooking.
Heat olive oil in a large pot over medium heat and sauté onion, leek, and shallots until softened.
Add minced garlic and lemon thyme sprigs, cooking until fragrant.
Pour in the vegetable broth and bring to a simmer.
Add sliced zucchini and salt, cooking until tender, about 8-10 minutes.
Stir in the spinach, parsley, and basil; cook for 1 minute until wilted.
Immediately transfer the soup to a blender or use an immersion blender to process until perfectly smooth.
Place the pot or blender container into your prepared ice bath to cool the soup rapidly, maintaining its bright green color.
Once chilled, taste and adjust with lemon juice, salt, and black pepper.
Serve in bowls garnished with thinly sliced turnips and summer squash, then drizzle with a touch of extra-virgin olive oil.