Chicken with Mustard Mascarpone Marsala Sauce (what a mouthful!)

Season chicken with salt and pepper. Heat olive oil in a heavy large skillet over high heat. Add chicken and cook just until brown, about 4 minutes per side. Transfer chicken to a plate and cool slightly. While chicken cools, melt 2 tablespoons of butter in the same skillet over medium-high heat, then add onion and saute until tender, about 2 minutes. Add mushrooms and garlic and saute until the mushrooms are tender and the juices evaporate, about 12 minutes. Add wine and simmer until it is reduced by half, about 4 minutes. Stir in mascarpone and mustard. 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. Stir in chopped parsley. Season the sauce, to taste, with salt and pepper.