First and foremost, preheat the oven to 350F. Whenever you are baking something, the oven has to be already hot when you put your dish inside or it will not cook evenly and the cooking time will be off.
Cook your chicken and shred it. Put two pots of boiling water, throw in your chicken in one and cook it for about 8 minutes. The other pot is for cooking the fettuccine.
Cook the fettuccine according to the package directions. Before draining it, reserve ½ cup of the cooking water to mix into the sauce.
While the chicken is boiling, and the pasta is cooking, slice the mushrooms and melt butter in a big skillet. Saute the mushrooms until they are cooked through and there is no water left in the bottom of the skillet. I like to slice them a little bit thick so you get a nice piece with every bite.
Now that everything is cooked, grab a large bowl and mix together 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. Be careful not to bake it for too long or it will dry out