If you have time to marinate the beef for a few hours, whisk all the sauce ingredients except the cornstarch. Add the beef strips to a freezer bag and pour the marinade over it. Refrigerate for up to 24 hours. When ready to cook, remove beef from marinade, add liquid to a bowl and whisk in cornstarch.
Reduce heat to medium and add the onion and peppers back to the skillet. Stir the sauce and add it to the skillet.
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)