For the sauce:
Mix together the soy sauce, dark soy, oyster sauce, rice vinegar, white pepper, and msg.
For the rest of the dish:
Cook the udon noodles according to package instructions.
In a large skillet or wok, heat the neutral oil over high heat.
Add the ground meat and cook, breaking it up with a wooden spoon, until browned, about 5 minutes.
Add the onion, garlic, carrots, cabbage, shiitake mushrooms, and choy sum. Stir-fry until the vegetables are tender-crisp, about 3-5 minutes.
Add the cooked udon noodles and the sauce. Toss everything together until the noodles are evenly coated and heated through, about 2-3 minutes.
Garnish with the scallions and serve immediately.