Chicken with Mustard Mascarpone Marsala Sauce

INGREDIENTS

• 5 tablespoons butter, divided
• 2 tablespoons olive oil
• 2 tablespoons Dijon mustard
• 1 cup dry Marsala wine
• 1 cup (8 ounces) mascarpone cheese
• 12 ounces dried fettuccine
• 1 pound cremini mushrooms, sliced
• Salt and freshly ground black pepper
• 3/4 cup chopped onion
• 2 tablespoons minced garlic
• 1 1/2 pounds boneless skinless chicken breasts, each breast cut crosswise into 3 pieces
• 2 tablespoons chopped fresh Italian parsley leaves, plus whole sprigs, for garnish