1. Melt the butter in a large pot over medium heat.
2. Add the onion and ¾ teaspoon salt and cook until onion is softened, about 6 minutes (do not brown.)
3. Add the parsnip, rutabaga, celery root, thyme and broth.
4. Bring to a boil and reduce heat. Cover and simmer until the vegetables are tender, 15 to 20 minutes.
5. Discard the thyme. Puree the soup in batches using an immersion blender until smooth.
6. Stir in the Florida Orange Juice, ginger and water (plus more to thin to desired consistency).
7. Season with salt and garnish with tarragon and pepper.
8. Serve immediately.