In a bowl, combine 1 minced garlic clove, 1 teaspoon sesame oil, 1 tablespoon soy sauce, 1 teaspoon oyster sauce, 1 teaspoon vinegar, and 1 teaspoon cornstarch. Add 200 grams of thinly sliced beef, ensuring it's well-coated with the marinade. Let it sit for about 10 minutes.

In a pot, bring water to a boil and add the udon noodles. Cook for 1-2 minutes until just tender; drain and set aside.

Heat a large skillet or wok over medium-high heat; sauté 1 tablespoon of minced garlic and sliced onion until fragrant and translucent, about 2 minutes. Add Chinese cabbage; stir-fry until wilted, about 2-3 minutes.

Add the marinated beef to the same pan and stir-fry for 2-3 minutes until browned and no longer pink. Cook in batches if necessary.

In a small bowl, mix together 3 tablespoons dark soy sauce, 1 tablespoon oyster sauce, 2 teaspoons fresh ground black pepper, and 3 tablespoons water. Pour into the skillet and cook over low heat until the sauce thickens, about 1-2 minutes.

Return cooked udon noodles and sautéed vegetables to the skillet; toss together ensuring everything is well coated in the sauce. Cook for an additional minute.