3/4 cup chopped onion
1 pound cremini mushrooms, sliced
2 tablespoons minced garlic
1 cup dry Marsala wine
1 cup (8 ounces) mascarpone cheese
2 tablespoons Dijon mustard
2 tablespoons chopped fresh Italian parsley leaves, plus whole sprigs, for garnish
12 ounces dried fettuccine
Melt 2 tablespoons of butter to a 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. Simmer, uncovered, over medium-low heat until 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.