Marinating the beef in a mixture of egg white, starch, salt and pepper (a technique referred to as "velveting" in Chinese cooking) gives the meat a unique texture.
Prepare the marinade by whisking the egg white, corn starch, salt and pepper in a bowl.
Add the beef to the marinade. Mix well.
Cover the bowl and allow the beef to marinate for 30 minutes.
Make the batter by mixing the flour and corn starch with enough milk to make a thick but still pourable paste.
Make the sauce by stirring together all the ingredients.
Heat enough cooking oil in a wok or frying pan to reach a depth of at least two inches.
Add the beef to the batter and mix to coat each piece well.
Drop the beef slices one by one into the hot oil and fry until lightly golden and crisp. Depending on the size of your wok or frying pan, you may need to do this in batches to prevent overcrowding.
Drain the fried beef in a strainer or a stack of paper towels.
Pour off the oil leaving only about two tablespoonfuls.
Stir fry the bell peppers and sliced onion with a generous pinch of sugar until the onion slices start to caramelize.
Add the cooked beef back into the wok.
Pour in the sauce.
Stir fry for half a minute until the beef pieces are glistening with the sauce.