Slice the beef thinly. Marinate with minced garlic, julienned onion, a little seasoning, oyster sauce, ground black pepper, and cooking oil. Cut the beef meatballs into quarters.

Trim the mushroom stems. Cut the green onions and cilantro into short pieces.

Cook the udon noodles according to the package instructions. Rinse with cold water and drain.

Heat a pan over high heat, add cooking oil, and stir-fry the beef until just cooked. Add the mushrooms and stir-fry until just tender, then remove from heat.

Pour the broth into a pot and bring to a boil. Season to taste. Add the udon noodles and beef meatballs. Bring to a boil again, then add the green onions and cilantro. Turn off the heat.

Transfer the noodles to bowls, top with the stir-fried beef and mushrooms, and ladle the hot broth over.