1. Heat oil in a large pot. Add chopped onions and sauté for three minutes. Add leeks and sauté another three minutes, until just starting to caramelize.
2. Add water, broth, whole onion, and turkey. Bring to a boil.
3. Cook on a soft rolling boil for 90 minutes, skimming the top to ensure it stays clear. (You can skip this step if you don’t care if the soup is cloudy; it still tastes great.)
4. Remove the whole onion and discard. I remove the turkey, shred it, and return it to the soup, but you can also just feed it to your kids, who are probably complaining that there’s nothing to eat!
5. Add salt, pepper, and celery roots and cook for 10 minutes. Add carrots and cook another 15 minutes. Add zucchini and cook five to eight minutes more. (I slightly undercook the zucchini so it doesn’t get mushy by the time I want to serve the soup.)
I wouldn’t recommend freezing this soup, as the celery roots, carrots, and zucchini will get waterlogged, but you can make it in advance without the vegetables, and add some freshly cooked vegetables before serving.