Roast Chicken with Wild Mushroom Sauce

INGREDIENTS

• 4 boneless chicken breast halves with skin (about 11 ounces each)
• Coarse salt and freshly ground pepper
• 2 tablespoons extra-virgin olive oil
• 1 small yellow onion, thinly sliced
• 1 pound assorted mushrooms (such as shiitake, cremini, and white), stemmed and quartered
• 2 tablespoons sherry vinegar
• 1 tablespoon all-purpose flour
• 1 1/2 cups homemade or low-sodium store-bought chicken stock
• 2 tablespoons chopped fresh flat-leaf parsley, plus sprigs
• 1 tablespoon heavy cream