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3 tablespoons unsalted butter

1 small onion, thinly sliced 

12 ounces mixed mushrooms, such as cremini, shiitake and oyster, stemmed and chopped 

Kosher salt and freshly ground black pepper 

4 cups chicken broth 

1 1/2 teaspoons Marsala, optional 

About 1/4 cup creme fraiche, for serving 

1 tablespoon sliced chives (1/2-inch pieces)

Olive oil, for drizzling