In large bowl, combine beef, soy sauce oyster sauce, salt, and pepper. Mix well. Let is stand for 10 minutes.
Add the cornstarch. Continue to mix until all the ingredients are well blended. Set aside.
Meanwhile, heat a pan. Spray or pour cooking oil. Once the oil becomes hot, add the onion and peppers. Stir fry for 2 to 3 minutes.
Transfer the cooked onion and peppers on a plate. Set aside.
Add oil to the same pan. Put-in the beef. Stir fry until the beef turns medium brown (around 7 to 8 minutes). Make sure that you are cooking in very high heat. Add more cooking time if needed.
Add the cooked onion and peppers. Continue to stir fry for 1 minute.
Transfer to a serving plate.
Serve with rice. Share and enjoy!