This hearty soup is chock-full of sweet and mellow winter root vegetables such as rutabagas, celery root, parsnips and carrots, their flavors heightened and concentrated by roasting before they're added to the soup. Salsify, turnips, potatoes and sweet potatoes also work beautifully in this soup, so feel free to use whatever you like or happen to have on hand.
The soup's foundation is chicken stock (although you can also use vegetable stock); make sure yours is flavorful, not salty.
If you want a more substantial dish, add cubes of roast chicken or turkey. To serve, drizzle the soup with truffle oil or top with a few shavings of Parmesan cheese, if desired.
Position a rack in the middle of the oven and preheat the oven to 400 degrees F. Toss the carrots, parsnips and rutabaga with 2 tablespoons of olive oil and spread the vegetables in a single layer on a rimmed baking tray. Season with salt and pepper, and roast until the vegetables are tender and have begun to caramelize, turning them with a spatula halfway through.
Meanwhile, bring a pan of water to a boil and add 1 teaspoon of salt and the celery root. Cook at high heat until the celery root is tender, 10 to 15 minutes. Drain and spread the cubes of celery root in a single layer to cool.
Heat the remaining 2 tablespoons of oil in a large stock pot. Add the onion and cook over medium heat until the onions are tender and have begun to color, about 15 minutes. Add the garlic and cook another 3 or 4 minutes, stirring frequently. Watch carefully so the garlic doesn't burn.
Add the chicken stock to the pan and bring to the start of a simmer over high heat. Add the roasted vegetables and the celery root to the pan and reduce the heat to medium. Cook until the vegetables are hot. Stir in the tarragon, and season the soup to taste with salt and pepper. Garnish each serving with a drizzle of truffle oil or a sprinkling of Parmesan curls, if desired.