Make the sauce by combining the soy sauce, dark soy, oyster sauce, rice vinegar, white pepper, and MSG.
Heat the neutral oil in a wok or large skillet over high heat.
Add the ground meat and cook, breaking it up with a wooden spoon, until browned, about 3-4 minutes.
Add the onion, garlic, carrots, cabbage, mushrooms, and choy sum. Stir-fry until the vegetables are tender-crisp, about 3-4 minutes.
Add the cooked udon noodles and the sauce. Toss everything together until the noodles are heated through and the sauce is evenly distributed, about 2-3 minutes.
Garnish with the scallions and serve immediately.
