How to Make It
Heat oil in a large pot over high heat. Add mushrooms, pancetta, celery, leek, and garlic. Cook, stirring often, until vegetables have softened, about 6 minutes. Add wine if using and cook 1 minute. Add rosemary, salt, pepper, broth, and 1 cup water, then cover and cook until simmering, about 6 minutes.
Whirl soup in a blender until smooth. Divide among 4 bowls and top each with a dollop of crème fraîche and a sprinkle of chives and hazelnuts.