In a mixing bowl, combine minced garlic, sesame oil, soy sauce, oyster sauce, rice vinegar, and corn starch to create a thick marinade. Add the thinly sliced beef, ensuring every piece is well-coated. Allow the beef to marinate at room temperature for about 10 minutes.

Bring a pot of water to a boil and add the udon noodles. Cook for 1-2 minutes, or until they are just tender yet still chewy. Drain the noodles and set them aside in a colander.

Heat a large wok or skillet over medium-high heat and add a splash of oil. Once hot, add the minced garlic and sliced onion, stirring for about 1 minute until fragrant and browned. Toss in the chopped Chinese cabbage with salt and sugar, stir-frying for 2-3 minutes. Set it aside.

In the same wok, add more oil if needed and heat over high heat. Add the marinated beef, spreading it out for even cooking. Stir-fry for about 2-3 minutes until browned.

Lower the heat slightly and pour the dark soy sauce, oyster sauce, and water into the same hot wok. Stir in fresh ground black pepper and potato starch, simmering for 2-3 minutes until the sauce thickens.

Return the cooked beef and drained udon noodles to the wok with the black pepper sauce. Toss gently for about 1-2 minutes until coated. Add the vegetables back to the wok and give a final stir.