Melt 1/4 cup butter in a large skillet; add chicken pieces, and cook until browned. Remove chicken from skillet, reserving pan drippings in skillet; set chicken aside. Add mushrooms and next 5 ingredients to pan drippings in skillet, and saute until vegetables are tender. Add chicken; reduce heat, and cook 15 minutes or until chicken is done and mixture is thoroughly heated. Set aside, and keep warm. Cook fettuccine according to package directions, omitting salt; drain. Place fettuccine in a large bowl. Combine half-and-half, 1/2 cup melted butter, parsley, 1/4 teaspoon salt, and 1/4 teaspoon pepper; stir well. Add half-and-half mixture to fettuccine; toss gently to combine. Add chicken mixture and Parmesan cheese to fettuccine; toss gently to combine. Sprinkle with pecans, and serve immediately.