Marinate the Beef. In a medium bowl, combine minced garlic, sesame oil, soy sauce, oyster sauce, vinegar, and cornstarch slurry. Add the thinly sliced beef and toss well to coat. Let it marinate for at least 10 minutes while you prepare the other ingredients — 30 minutes is even better for a more tender texture.

Cook the Udon Noodles. Bring a pot of water to a boil. Add the udon noodles and cook for just 1–2 minutes (or as directed on the package). You want them loosened but still bouncy. Drain and rinse briefly under warm water, then set aside.

Stir-Fry the Vegetables. Heat a tablespoon of oil in a large wok or skillet over medium heat. Add minced garlic and sliced onions, stir-frying until fragrant and translucent. Toss in the cabbage (or your chosen greens) and cook until wilted. Season lightly with a pinch of salt and sugar, then transfer the vegetables to a plate and set aside.

Cook the Beef. In the same wok, add a touch more oil if needed. Add the marinated beef and stir-fry for 2–3 minutes until browned and just cooked through. Avoid overcooking — the beef should remain juicy and tender. Remove from the wok and set aside.

Prepare the Black Pepper Sauce. Combine dark soy sauce, oyster sauce, black pepper, water, and potato starch in a small bowl. Whisk until smooth. Pour this mixture into the same wok over low heat. Stir continuously until the sauce thickens and becomes glossy.

Combine Everything. Add the cooked beef and udon noodles into the sauce. Toss well to coat every strand of noodle and piece of beef evenly. Add back the sautéed vegetables, stir everything together one last time, and let it cook for another 1–2 minutes to heat through.

Serve. Transfer to serving bowls and enjoy hot! Garnish with a sprinkle of extra black pepper or sesame seeds for a finishing touch.