In a medium bowl, mix minced garlic, sesame oil, soy sauce, oyster sauce, Chinese white rice vinegar, and cornstarch slurry until well combined. Add the thinly sliced beef and coat it thoroughly in the marinade. Let it rest for about 10 minutes.

Bring a large pot of water to a rolling boil over high heat. Once boiling, add the udon noodles and cook for 1-2 minutes, or just until they are loosened and tender. Drain the noodles in a colander and rinse them briefly under cold water.

Heat a wok or large skillet over medium heat and add a splash of oil. Once the oil shimmers, add the minced garlic and sliced onion, stirring frequently, for about 2-3 minutes until they soften.

In the same hot wok, add the marinated beef, spreading it out in a single layer. Stir-fry for 2-3 minutes, or until the beef is browned and cooked through. Remove the beef from the wok.

In a separate bowl, whisk together dark soy sauce, oyster sauce, ground black pepper, water, and potato starch until smooth. Pour this mixture into the hot wok, stirring constantly over low heat for 2-3 minutes until the sauce thickens.

Add the cooked udon noodles and stir-fried beef back into the wok with the sauce. Toss everything together, ensuring the noodles and beef are well coated. Return the wilted vegetables to the pan and serve hot.