1. Slice beef flank steak. Combine marinade ingredients. Marinate beef in a bowl for at least 30 minutes. Cut peppers and onion into bite-size pieces. Set aside.
2. Mix the sauce ingredients in a bowl and set aside.
3. Place approximately 1 cup of cooking oil in a heated wok or fry pan and heat oil to medium heat. Place marinated beef in heated oil and cook until almost done. Drain beef into a strainer. Set aside.
4. Leave about a teaspoon of oil in the wok. Place it back on the stove. Stir-fry onion and peppers for 30 seconds. Pour sauce mixture over onions and green peppers. Stir occasionally with a spatula.
5. When sauce mixture begins to darken in color, place beef back into the wok and stir-fry swiftly. The dish is done when the peppers have softened. Serve hot with steamed white rice.