Lightly coat chicken with flour. In a large skillet, brown chicken in 4 tablespoons butter. Transfer to a greased 13-in. x 9-in. baking dish. Cover and bake at 350° for 35-40 minutes or until juices run clear. Meanwhile, saute mushrooms and onion in the remaining butter until tender; set aside.
In a saucepan, bring broth to a boil over medium heat. Reduce heat; add rosemary and pepper. Simmer, uncovered, for 20 minutes or until reduced by half. Combine cornstarch and water until smooth; stir into broth. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in mushroom mixture. Serve chicken over fettuccine; top with mushroom sauce.