In a mixing bowl, whisk together minced garlic, sesame oil, soy sauce, oyster sauce, vinegar, and the starch slurry until well combined. Add the thinly sliced beef, ensuring each piece is coated in the marinade. Let it rest for about 10 minutes.

Bring a pot of salted water to a rolling boil, then add the udon noodles. Cook them for 1-2 minutes until tender but still chewy. Drain the noodles in a colander and set aside.

Heat a large wok or skillet over medium-high heat and add a splash of oil. Add minced garlic and sliced onion, sautéing for about 1 minute until fragrant. Add the Chinese cabbage, sprinkle with a pinch of salt and sugar, and stir-fry for another 2-3 minutes until the cabbage is wilted and tender. Remove from the pan and set aside.

In the same pan, add a little more oil if needed and add the marinated beef. Stir-fry for 2-3 minutes until browned and cooked through. Remove from heat, leaving any drippings in the pan.

Combine the drippings from the beef with dark soy sauce, oyster sauce, black pepper, and water in the wok. Cook this mixture over low heat, stirring constantly for about 1-2 minutes, until it thickens slightly.

Add the drained udon noodles and stir-fried vegetables back into the pan. Toss everything gently for a few minutes until well-coated with the savory sauce.