Preheat oven to 350°F (175°C) and grease a 9x13 inch baking dish.
Cook spaghetti in a large pot of boiling salted water until al dente, drain and set aside.
Melt butter in a large skillet over medium heat and sauté sliced mushrooms until tender and browned.
Whisk in flour and cook for 1 to 2 minutes to create a light roux.
Gradually whisk in chicken broth and heavy cream; simmer until the sauce thickens, about 5-7 minutes.
Stir in dry sherry, Parmesan cheese, salt, and pepper until the cheese is melted and sauce is smooth.
In a large mixing bowl, combine the cooked spaghetti, shredded chicken, and sauce, tossing thoroughly to coat.
Transfer the mixture into the prepared baking dish.
Sprinkle breadcrumbs and mozzarella cheese evenly over the surface.
Bake for 30 minutes until the sauce is bubbly and the topping is golden brown.
Garnish with fresh parsley and allow to rest for 5 minutes before serving.