In a small bowl, mix together all the sauce ingredients (oyster sauce, soy sauce, dark soy sauce, sugar, chicken bouillon powder, and white pepper). Add a spoonful of water to help loosen the sauce and make it easier to mix. Set aside.
Thinly slice the char siu and set aside.
In a large pan over medium-high heat, add the sliced onions. Stir fry until lightly browned. Then add the white parts of the green onions and cook for 1 more minute.
Next add the sliced cabbage and stir fry for 2-3 minutes until slightly softened. Add the bean sprouts and cook for 1 more minute.
Slide all the vegetables onto a plate and set aside.
In the same large pan over medium-high heat, add some more oil. Then add all the chow mein noodles followed by ¼ cup of water. Mix, mix, mix (I like to use cooking chopsticks for this) to cook the noodles for about 2 minutes. Let it get golden and crisp. Drizzle the sauce mixture (prepared in Step 1) over the noodles. Mix evenly.
Then add all the vegetables back to the pan and mix evenly.
Add the chopped char siu and the green parts of the green onions. Stir to mix. Then the dish is ready to serve! Eat and enjoy ^_^