Cut the beef steak into 1cm wide strips. Marinate with all the marinating seasonings including soy sauce, oyster sauce, pepper, sugar, Chinese cooking wine and cornstarch for 15 minutes. Mix in sesame oil just before start frying.
Add cooking oil in a large pan until the oil can evenly cover the bottom of the pan. spread the beef strips in. Let them stay for around 5 to 6 seconds and gently stir them several times until the beef strips change color. Transfer the beef out and leave around 1 tablespoon of the oil in.
Fry garlic until aromatic. Fry green peppers and red onion until slightly soft (do not kill them). Add a small pinch of salt. Return beef strips back. Mix well. Spread freshly ground black pepper.