In a bowl, whisk together minced garlic, sesame oil, soy sauce, oyster sauce, white rice vinegar, and cornstarch slurry.
Coat the thinly sliced beef with the marinade and let sit for 10 minutes.
Boil udon noodles in a large pot for 1 to 2 minutes until slightly loosened, then drain and set aside.
In a small bowl, combine dark soy sauce, oyster sauce, fresh ground black pepper, water, and potato starch to create the black pepper sauce.
Heat oil in a wok over medium heat and stir-fry garlic and onions until softened.
Add the marinated beef to the wok and cook for 2 to 3 minutes until browned.
Pour in the black pepper sauce and stir over low heat until the sauce thickens and becomes glossy.
Add cooked udon noodles and Chinese cabbage to the wok, stirring to coat everything evenly with the sauce.
Toss together and serve immediately.