Celery Root Soup with Truffle Oil

Name: Celery Root Soup with Truffle Oil
Ingredients: 1 Tbs. extra-virgin olive oil; 1 1/2 cups (7 1/2 oz./235 g) finely chopped yellow onion; 1 parsnip, peeled and chopped into 1/2-inch (12-mm) pieces; 1 celery stalk, thinly sliced; 2 tsp. finely chopped garlic; 3 cups (24 fl. oz./750 ml) vegetable broth; 1 celery root, (about 14 oz./440 g), peeled and chopped into 1/2-inch (12-mm) pieces; 1 large Yukon Gold potato, about 1/2 lb. (250 g), peeled and cut into 1/2-inch (12-mm) pieces; 1 tsp. dried savory or thyme; 1/4 tsp. ground nutmeg; Sea salt and freshly ground pepper; Truffle oil for drizzling (optional); 1 Tbs. finely chopped fresh chives (optional)