In a medium bowl, combine beef, soy sauce, cooking wine, ground black pepper, garlic powder, and cornstarch. Mix well and marinate for 30 minutes.
Heat a wok and then pour-in the cooking oil. Stir fry the marinated beef in batches of 2 or 3 for 3 minutes. Cooking in batches preserves the heat which is essential in stir fry; it also cooks the meat equally. Place the cooked beef in a plate. Set aside.
Meanwhile, using the remaining oil, stir fry the onion and bell peppers for 2 minutes.
Add-in the cooked beef. Toss and stir fry for 1 to 2 minutes more adding salt and soy sauce as needed.
Transfer to a serving plate. Serve!
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