1. Preheat the oven to 350°F. Cook fettuccine according to package directions and drain.
2. In a large bowl, combine the chicken, dressing, chicken broth, and mushrooms and stir until well mixed. Pour into a 2-quart casserole dish.
3. In a food processor, add the bread and Italian seasonings and pulse while adding the melted margarine. Top the chicken evenly with the breadcrumbs. Bake for 20 minutes or until bubbly around edges.