Food Network teaches how to crush, slice and mince garlic. Peel off some of the papery skin from the garlic and then smash the head of garlic with the heel of your hand to loosen the cloves; if you only need a few cloves, leave the head intact and pull some off. Separate the cloves. To peel a clove, cut away the root end with your knife. Lay the flat side of the knife over the clove while holding the knife handle, then with the heel of your free hand carefully whack the knife against the garlic to separate the skin from the clove. To crush the peeled garlic, lay the flat side of the knife over the clove and smash it again. To slice peeled garlic, lay the clove flat on the cutting board and hold it with the fingertips of one hand, keeping them curled under. Using a rocking motion with the knife, make thin slices by moving the knife slowly across the clove. To mince peeled garlic, lay the flat side of a knife over the clove and smash it. Roughly chop the clove then move your free hand flat across the tip of the knife and use a rocking motion to chop the garlic until it’s finely minced.
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.
Meanwhile, bring a large pot of salted water to a boil. Add the fettuccine and cook until al dente, stirring occasionally, about 8 minutes. Drain. Toss the fettuccine with 3 tablespoons of butter and season, to taste, with salt and pepper. Swirl the fettuccine onto serving plates. Spoon the chicken mixture over top. Garnish with parsley sprigs and serve.