In a medium bowl, combine the sliced beef with 1 tablespoon soy sauce, 1 tablespoon cornstarch, rice vinegar, and sesame oil. Toss until evenly coated and let marinate for 10–15 minutes while you prep the vegetables.
In a small bowl or measuring cup, whisk together the remaining 2 tablespoons soy sauce, 1 tablespoon cornstarch, beef broth, oyster sauce, brown sugar, grated ginger, and minced garlic. Set aside.
Heat 1 tablespoon vegetable oil in a large skillet or wok over high heat. Add the onion chunks and bell pepper strips. Stir-fry for 4–5 minutes until they are crisp-tender and slightly charred. Remove from the pan and set aside.
Add the remaining 1 tablespoon oil to the same pan. Once hot, add the marinated beef in a single layer. Let sear undisturbed for 1 minute, then stir-fry for 2–3 minutes until just browned but not fully cooked through. Remove the beef and set aside with the vegetables.
Reduce heat to medium. Give the sauce a quick stir (cornstarch settles) and pour it into the pan. Cook, stirring constantly, until the sauce thickens and becomes glossy, about 1–2 minutes.
Return the beef and vegetables to the pan. Toss everything together until well coated in the sauce and heated through, about 1–2 more minutes. Season with black pepper.
Garnish with sliced green onions. Serve immediately, directly from the pan, over steamed rice or noodles.