Preheat the oven to 350F.
Cook the fettuccine according to the package directions. Before draining it, reserve ½ cup of the cooking water to mix into the sauce.
Melt butter in a large skillet over medium heat and saute the mushrooms until they are cooked through and there is no water left in the bottom of the skillet.
In a large bowl combine the pasta, shredded chicken, sauteed mushrooms, condensed cream of chicken soup, sour cream, pasta water, parsley, salt, and pepper.
Pour the mixture into a greased 9x13-inch casserole dish, sprinkle the Parmesan cheese, and bake for about 30 mins or until it is hot and bubbly.