In a large skillet or wok, heat the vegetable oil over medium-high heat. Add the sliced beef and cook for about 3-4 minutes until browned. Remove the beef from the skillet and set aside.

In the same skillet, add the onion and red bell pepper. Stir-fry for about 3-4 minutes until the vegetables are tender.

Add the minced garlic and ginger to the skillet and cook for an additional 1 minute until fragrant.

Return the cooked beef to the skillet. Add the udon noodles, soy sauce, oyster sauce, black pepper, and sesame oil. Toss everything together and cook for another 2-3 minutes until heated through.

Season with salt to taste and garnish with chopped green onions before serving.