Place the sliced beef and cornstarch in a medium sized bowl. Mix and set aside until ready to use.
Preheat a wok (or large skillet) to high and then add 2 tbsp of oil. Place the onions, red and yellow bell peppers in the wok and cook for 1-2 minutes. Once slightly translucent; Remove them from the wok and set aside.
Place the beef in the wok. Stir-fry for 3-5 minutes until almost fully cooked.
Turn the heat down to medium. Add the sauce to the meat and stir-fry for 1 minute.
Finally, add the bell peppers and onions and stir-fry in the wok for an additional 30 seconds.