In a mixing bowl, whisk together garlic, sesame oil, soy sauce, oyster sauce, white rice vinegar, and corn starch until smooth. Add thinly sliced beef and coat thoroughly. Let marinate for 10 minutes.

Bring a pot of water to a boil and cook udon noodles for 1–2 minutes until slightly softened. Drain and rinse with cold water, then set aside.

Heat 1 tablespoon of oil in a wok over medium-high heat. Add minced garlic and sliced onions and stir for 1-2 minutes until fragrant. Toss in chopped Chinese cabbage and stir-fry for 2-3 minutes until wilted.

Add marinated beef to the wok, cooking for 2-3 minutes until browned and cooked through. Remove beef and set aside.

In the same wok, scrape the browned bits, then add dark soy sauce, oyster sauce, fresh ground black pepper, and water. Bring to simmer and add corn starch, stirring until sauce thickens (2-3 minutes).

Return udon and sautéed beef to the wok, tossing for 1-2 minutes until everything is well-coated and heated. Serve hot.