1. Melt butter in a large pot over medium heat. Add onion and 3/4 teaspoon salt, and cook until onion is softened, about 6 minutes (do not let brown). Add parsnip, rutabaga, celery root, thyme, and broth. Bring to a boil, and reduce heat. Cover, and simmer until vegetables are tender, 15 to 20 minutes.

2. Discard thyme. Puree soup in batches in a blender until smooth (fill blender only halfway), and transfer to a clean pot.

3. Finely grate enough peel of 1 orange to yield 1/2 teaspoon zest. Working over a bowl, cut segments of orange free of membranes, and set aside. Squeeze juice from membranes into a bowl, then squeeze enough juice from remaining orange to yield 1 cup total. Just before serving, reheat to warm. Stir in orange zest, juice, ginger, and water, plus more to thin to desired consistency. Season with salt, and garnish with orange segments, tarragon, and pepper.