Boil water in a large pot, add udon noodles, and cook according to package instructions (usually about 8-10 minutes). Once cooked, drain and set aside.
In a mixing bowl, combine sliced beef, 1 tablespoon of soy sauce, 1 tablespoon of black pepper, garlic, and ginger. Toss well and let it marinate for about 10 minutes.
Heat vegetable oil in a large wok or frying pan over high heat. Add the marinated beef and stir-fry for about 3-4 minutes until browned and cooked through. Remove the beef from the pan and set aside.
In the same pan, add the broccoli and sliced red bell pepper. Stir-fry for about 3-5 minutes until the vegetables are tender yet crisp.
Return the cooked beef to the pan with the vegetables. Add the cooked udon noodles, remaining soy sauce, oyster sauce, sesame oil, and the rest of the black pepper. Toss everything together until well combined and heated through.
Serve the Beef Black Pepper Udon hot, garnished with sliced green onions and a sprinkle of sesame seeds on top.
Notes
For extra heat, add more black pepper or chili flakes.