Whisk ingredients together in a small bowl and refrigerate for at least 30 minutes.
Heat oil in a large frying pan over medium heat.
Add beef and saut for 3-4 minutes just to brown it on all sides.
Remove from pan and set aside.
Add vegetables and saut for 5 minutes more, until beginning to soften.Stir in teriyaki sauce and allow mixture to come to a simmer.
Add beef and any drippings that have accumulated on the plate. Cook for 5 minutes, stirring occasionally.In a small bowl, combine cornstarch with 2 tsp cold water to dissolve. Stir into the pan of beef and vegetables. Allow the mixture to simmer and thicken for 2 minutes. Toss to evenly coat vegetables and beef in teriyaki sauce.