1Cook and drain noodles as directed on package.
2Meanwhile, diagonally slice onions; set green tops aside. In 12-inch nonstick skillet, heat oil over medium-high heat. Cook mushrooms, carrots, peppers and white portion of onions in oil 5 minutes, stirring occasionally, until soft. Remove mixture from skillet to bowl; cover to keep warm.
3In same skillet, cook beef 1 to 2 minutes or just until meat begins to brown; drain. In small bowl, mix teriyaki marinade and cornstarch until blended. Stir teriyaki mixture and mushroom mixture into skillet. Heat to boiling over medium heat, stirring constantly, until sauce is thickened.
4In large serving bowl, toss noodles with beef mixture and reserved green onion tops. Serve immediately.