Whisk minced garlic, sesame oil, soy sauce, oyster sauce, vinegar, and cornstarch slurry in a bowl. Coat thinly sliced beef and let it sit for 10 minutes while prepping other ingredients. The beef will absorb the flavors and become tender.
In a large pot of boiling water, cook the udon noodles for 1-2 minutes, until slightly loosed-up but not overcooked, of course! Drain and set aside. No overcooking's necessary in this recipe-you want it still to retain just that little bit of bouncy chew!
Scrape all that dreg into a small bowl with the dark soy, oyster sauce, black pepper, water and potato starch.
Heat a wok on medium, add oil, and stir-fry garlic and onions until soft. Cook the marinated beef for 2-3 minutes until browned. Then add black pepper sauce. Pour into a pan, stirring over low heat until it thickens and becomes glossy.
Add the cooked udon and Chinese cabbage to the sauce, stirring to coat. Toss everything together and serve.
