1 1/2 lbs boneless skinless chicken breasts, each breast cut crosswise into 3 pieces
Salt and freshly ground black pepper
2 T olive oil
5 T butter, divided
3/4 c chopped onion
1 lb cremini mushrooms, sliced
2 T minced garlic
1 c dry Marsala wine
8 oz mascarpone cheese
2 T Dijon mustard
2 T chopped fresh Italian parsley leaves, plus whole sprigs, for garnish
12 oz dried fettuccine
Sprinkle the chicken with salt and pepper. Heat the oil in a heavy large skillet over high heat. Add the chicken and cook just until brown, about 4 minutes per side. Transfer the chicken to a plate and cool slightly.
While the chicken cools, melt 2 tablespoons of butter to the same skillet over medium-high heat, then add the onion and saute until tender, about 2 minutes. Add the mushrooms and garlic and saute until the mushrooms are tender and the juices evaporate, about 12 minutes. Add the wine and simmer until it is reduced by half, about 4 minutes. Stir in the mascarpone and mustard. Cut the chicken breasts crosswise into 1/3-inch-thick slices. Return the chicken and any accumulated juices to the skillet. Simmer, uncovered, over medium-low heat until the chicken is just cooked through and the sauce thickens slightly, about 2 minutes. Stir in the chopped parsley. Season the sauce, to taste, with salt and pepper.