2 pounds boneless skinless chicken breasts, each breast cut crosswise into 3 pieces
Salt and freshly ground black pepper
Onion, large, halved and sliced
2 pound cremini mushrooms, quartered
2 cups dry Marsala wine
16 ounces mascarpone cheese
4 tablespoons Dijon mustard
4 tablespoons chopped fresh Italian parsley leaves
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 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.
Meanwhile, bring a large pot of salted water to a boil. Add the fettuccine and cook until al dente; if fresh about 2 minutes. Swirl the fettuccine onto serving plates. Place 2-3 chicken pieces on fettucine and spoon the mushroom mixture over top. Garnish with parsley.