Stir fry beef in 2 teaspoons oil for 1-2 minutes or until no longer pink. Remove and keep warm.
In the same pan stir fry broccoli in remaining oil for 4-5 minutes or until crisp-tender. Add onions; stir fry for 1-2 minutes. Return beef to pan.
Stir cornstarch mixture and gradually add to pan. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Reduce heat to low; cover and cook for 5-6 minutes or until meat and vegetables are tender.