Bring a large pot of salted water to a rolling boil. Add the udon noodles and cook according to package instructions until tender, about 6-8 minutes. Drain and rinse under cold water to stop cooking.

In a medium bowl, combine the thinly sliced beef with soy sauce and freshly ground black pepper, mixing well. Let marinate for about 10-15 minutes.

Heat vegetable oil in a large skillet over medium-high heat. Add minced garlic and sauté for about 30 seconds until fragrant. Add the marinated beef and cook for 3-4 minutes until browned.

Lower the heat to medium, add the drained udon noodles, and toss with the beef to coat in the sauce. Stir-fry for another 2-3 minutes.

Sprinkle chopped green onions over the dish, toss gently, and serve hot in bowls, offering extra black pepper on the side.