How to Make It
In a large pot of boiling salted water, cook the fettuccine until al dente.
Meanwhile, in a large, deep skillet, heat the olive oil. Add the onion and garlic and cook over moderate heat until fragrant, about 2 minutes. Add the mushrooms and prosciutto in an even layer, season with salt and pepper and cook undisturbed over moderately high heat until browned on the bottom, about 3 minutes. Cook, stirring, until the mushrooms are tender, about 3 minutes longer.
Drain the fettuccine, reserving 1 cup of the pasta cooking water. Add the fettuccine to the skillet. Stir in the mascarpone and the reserved pasta cooking water and toss the pasta over low heat until the sauce is creamy. Stir in the 1/4 cup of Parmesan and season the fettuccine with salt and pepper. Transfer the pasta to shallow bowls, sprinkle with the chopped parsley and serve with extra Parmesan cheese.