Slice the beef into strips (against the grain). Mix with the ingredients for the marinade. Leave to rest for 10 minutes.
Mix all the ingredients for the sauce then set aside.
Heat up the oil in a wok over high heat. Sear the marinated beef until it turns brown (should still be pink inside). Transfer it to a separate plate.
Add onion and pepper to the remaining oil in the wok. Cook for a minute or so.
Put the beef back into the wok. Stir fry for another 1-2 minutes until the beef is fully cooked.
Pour in the sauce. Stir until it becomes thick enough to coat the beef and vegetables nicely.
Sprinkle more ground black pepper for more intense flavour (optional). Serve hot with steamed rice.
Notes
1. Flank steak and skirt steak are the best cut of beef for quick stir-fry. Slicing it across the grain is the key to tenderness.2. Using a cheaper cut of beef (for braising) is not ideal but acceptable. You need to tenderize it before marinating:
Mix ½ teaspoon of baking soda with sliced beef then leave for at least 30 mins
Rinse the beef in 500ml / 2 cups of water with 4 teaspoons of vinegar. Then rinse again under running water