Firstly, marinade the sliced beef:- Combine the dark soy sauce (3-4 tablespoons), rice wine vinegar (3 tablespoons), diced garlic, chopped chilli, brown sugar (2-3 teaspoons), tomato paste, cracked black pepper together in a non metallic bowl and add the beef. Mix well to make sure all of the meat gets covered. Add more soya sauce if required.
Next, get all the vegetables ready and have everything required to hand.
Add the oil to the wok (on full heat), add the meat to the wok, cook for 1-2 minutes and remove from the pan and set aside.
Add more oil if necessary and add the veg that would take the longest to cook (green beans, carrots, broccolli). Add a splash of water to the pan if the wok starts to get dry.
Stir fry for a minute or two, next add the tomato, mushrooms, bean sprouts and sliced peppers. Pour in any remaining marinade sauce and add a tablespoon or two of rice wine vinegar. These will cook very fast, so check the veg with a fork (Stir fried veg should be firm).
Finally add the Pak choi, add back the beef strips along with any juices, stir fry for a minute or two until pak choi is softened.
Add a couple of tablespoons of oyster sauce and light soya sauce, cook for a minute or so to mix through.