Cook the pasta according to package directions; drain and keep warm. Coat a large skillet with cooking spray and heat on medium-high. Cook and stir beef, half at a time, for 1-2 minutes or until browned. Season with pepper. Remove and keep warm.
In the same skillet, saute mushrooms and onion in oil until tender. Stir in flour. Gradually add broth, stirring until blended. Bring to a boil; cook and stir for 2 minutes. Return beef to skillet and heat through. Serve over pasta. Top with green onions and sour cream.