In a bowl, marinate the beef with salt, soy sauce, dark soy sauce, sugar, and cornstarch. Mix well until the beef is evenly coated, then set aside while you prepare the other ingredients.

Bring a pot of water to a boil and cook the udon noodles for 1–2 minutes, just until they loosen. Drain, rinse under cold water to stop the cooking, and set aside.

In a small bowl, whisk together the sauce ingredients: soy sauce, dark soy sauce, oyster sauce, mirin, sugar, sesame oil, water, and cornstarch. Set aside.

Heat a wok or large skillet over medium-high heat. Add the marinated beef and stir-fry for 1–2 minutes, until browned and just cooked through. Remove from the pan and set aside.

In the same pan, add the garlic and sauté for about 20 seconds, until fragrant. Add the onion, carrot, cabbage, and mushrooms, and cook for 2–3 minutes, stirring often, until the vegetables soften slightly.

Add the cooked noodles, cooked beef, and the sauce mixture to the pan. Toss everything together for 30 seconds to 1 minute, making sure the noodles are coated evenly in the sauce.

Add the scallions and stir-fry for another 15 seconds to combine all the flavors.

Transfer to a serving plate and garnish with chopped scallions and sesame seeds. Serve immediately while hot.