In a bowl, mix beef, soy sauce, oyster sauce, salt and pepper together. Leave it for 10 minutes.
Add cornstarch and mix again all the ingredients until blended. Set aside.
In a wok, eat oil over medium heat.
Add onion and bell peppers. Cook for 3 minutes.
Transfer out the onion and bell peppers and place in a bowl/plate. Set aside.
With the remaining oil, add marinated beef. Cook for 9 minutes.
Add the cooked onion and bell peppers. Combine all ingredients and cook for another 2 minutes.
Serve and enjoy!
Inspired by Panlasang Pinoy.