In large pot of boiling water, cook noodles 5-6 minutes, until tender; drain, discarding liquid. Set aside; keep warm.
In medium nonstick skillet, cook beef over medium-high heat, stirring frequently, 3 minutes, until no longer pink. Remove beef from skillet. Wipe skillet clean; set beef aside.
Spray same skillet with nonstick cooking spray; heat. Add onions; cook over medium-high heat, stirring frequently, 7 minutes, until golden brown. Add mushrooms; cook, stirring frequently, 6 minutes, until lightly browned. Remove vegetables from skillet. Wipe skillet clean; set vegetables aside.
In same skillet, melt margarine; sprinkle with flour. Cook over medium-high heat,stirring constantly, until bubbling. Continuing to stir, gradually add broth, mustard, paprika, salt and pepper; cook, stirring constantly, 4 minutes, until mixture is thickened. Stir in sour cream.
Return cooked beef, onions and mushrooms to skillet; stir to coat. Cook, stirring frequently, until mixture is heated through.
Divide warm noodles evenly among 4 plates; top each portion of noodles with one-fourth of the beef mixture. Serve garnished with parsley.