In a mixing bowl, combine the minced garlic, sesame oil, soy sauce, oyster sauce, rice vinegar, and the starch slurry. Introduce the thinly sliced beef and gently turn it to coat evenly. Allow the beef to marinate for about 10 minutes.

Bring a large pot of water to a rolling boil. Add the udon noodles and cook them for about 1 to 2 minutes. Drain the noodles and rinse them under cold running water.

In a small bowl, blend the dark soy sauce, oyster sauce, freshly ground black pepper, water, and potato starch until fully dissolved.

Heat a wok over medium heat and add oil. Once hot, add the minced garlic and sliced onions, sauté them for about 2 to 3 minutes.

Introduce the marinated beef to the wok, spreading it out thinly and stir-fry until just browned.

Pour the prepared black pepper sauce around the perimeter of the wok, stirring gently until it thickens and becomes glossy.

Add the drained udon noodles and chopped Chinese cabbage to the wok. Toss everything thoroughly to combine and season with salt and sugar to taste.

Serve immediately, garnishing if desired.