Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13 inch baking dish.
Bring a large pot of lightly salted water to a boil. Add spaghetti, and cook for 8 to 10 minutes, or until al dente; drain.
Meanwhile, in a large saucepan, melt butter over low heat. Stir in flour, salt, and pepper. Cook, stirring, until smooth. Remove from heat, and gradually stir in chicken broth and cream.
Return to heat, and bring to a low boil for 1 minute, stirring constantly. Add sherry, then stir in cooked spaghetti, mushrooms, and chicken. Pour mixture into the prepared baking dish, and top with Parmesan cheese.
Bake 30 minutes in the preheated oven, until bubbly and lightly browned.