Marinate the Beef: In a bowl, combine sliced sirloin, soy sauce, oyster sauce, and cornstarch. Toss well and let marinate for 15 minutes.
Cook the Noodles: Bring a pot of water to a boil and cook spaghetti according to package instructions. Drain and set aside.
Sauté the Beef: Heat a large nonstick skillet or wok over medium-high heat. Add marinated beef and cook for 2–3 minutes until browned. Remove and set aside.
Cook the Veggies: In the same pan, add bell peppers and onion. Stir-fry for 2–3 minutes until slightly tender but still crisp.
Make the Sauce: In a bowl, mix soy sauce, oyster sauce, rice vinegar, hoisin, black pepper, sesame oil, honey, garlic, ginger, and water. Stir in the cornstarch slurry.
Bring It Together: Add cooked noodles and beef back to the pan with veggies. Pour in the sauce and toss everything together. Cook for another 1–2 minutes until the sauce thickens and coats everything.
Serve: Divide into bowls, add extra black pepper if desired, and serve hot.