Combine the ginger, brown sugar, corn starch, ground black pepper, and garlic powder in a bowl. Mix well.
Add the soy sauce and Sherry cooking wine. Stir.
Arrange the beef in a bowl. Pour half of the soy sauce mixture on the flank beef. Mix well. Marinate for 10 minutes.
Heat 2 tablespoons of cooking oil in a pan. Once the oil is hot, pan fry both sides of the marinated beef for 30 secs to 1 minute per side. Remove the pan fried beef and place it in a bowl. Set aside.
Meanwhile, add the remaining oil to the pan. Add the onion and broccoli. Stir-fry for 3 minutes.
Put-in the pan-fried beef. Continue to stir-fry for 2 to 3 minutes.
Pour-in the remaining soy sauce mixture. Stir an cook for 1 minute.
Add the oyster sauce. Stir and cook for a minute more.
Transfer to a serving bowl.
Serve. Share and enjoy!