In a large bowl, whisk together minced garlic, sesame oil, soy sauce, oyster sauce, and a corn starch slurry until smooth. Add the thinly sliced beef, ensuring each piece is well-coated, then cover and let it sit for 10 minutes at room temperature.

Bring a large pot of water to a rolling boil, then add the udon noodles. Cook them for 1-2 minutes until they are tender yet chewy. Drain and rinse under cold water.

Heat a tablespoon of cooking oil in a wok or large frying pan over medium heat. Add minced garlic and sliced onions, cooking for about 2 minutes until fragrant. Add the Chinese cabbage and cook for an additional 2-3 minutes until wilted.

In the same pan over medium-high heat, add the marinated beef mixture. Cook for 2-3 minutes, stirring frequently until the meat is browned. Transfer the cooked beef to a plate.

In a mixing bowl, combine dark soy sauce, oyster sauce, freshly ground black pepper, water, and potato starch. Pour into the hot pan and cook over low heat for about 2-3 minutes until the sauce thickens.

Add the cooked udon noodles and sautéed beef back into the pan with the sauce. Toss everything together gently, ensuring each noodle and beef slice is well-coated. Return the sautéed vegetables to the mix and stir for another minute.