Heat oil in large skillet over medium-high heat until hot. Add beef; cook and stir 5 to 6 minutes or until no longer pink. Drain. Add vegetables.
In small bowl, combine stir-fry sauce and cornstarch; mix well. Pour over vegetable mixture; stir to combine. Reduce heat; cover and simmer 6 to 8 minutes or until vegetables are crisp-tender, stirring occasionally.