1. Start by marinating the beef slices with soy sauce, oyster sauce, and black pepper. Let it sit for at least 15 minutes to absorb the flavors.
2. While the beef marinates, cook the udon noodles according to the package instructions. Drain and set aside.
3. In a large skillet, heat the vegetable oil over medium-high heat. Add the marinated beef and cook until browned on all sides, about 4-5 minutes. Remove the beef from the skillet and set it aside.
4. In the same skillet, add the red bell pepper, broccoli, and garlic. Stir-fry for about 3 minutes until the vegetables are tender-crisp.
5. Return the beef to the skillet, add the cooked udon noodles, and toss everything together. Drizzle sesame oil over the mixture and continue to stir-fry for another 2 minutes.
6. Adjust the seasoning with salt if needed. Garnish with chopped green onions and sesame seeds before serving.

Remember, cooking is not just about following steps; it’s about listening to your senses, much like my abuela taught me. Feel free to adjust the ingredients to suit your palate!