Marinate the Beef:
In a medium bowl, combine beef slices with soy sauce, cornstarch, and sesame oil. Toss to coat and let sit for at least 15 minutes.
Prepare the Sauce:
In a small bowl, whisk together beef broth, soy sauce, oyster sauce, cornstarch, brown sugar, garlic, ginger, and sesame oil. Set aside.
Cook the Beef:
Heat 1 tbsp vegetable oil in a large skillet or wok over medium-high heat. Add beef in a single layer (work in batches if necessary) and sear for 1–2 minutes per side until browned but not fully cooked. Remove beef and set aside.
Cook the Vegetables:
Add remaining 1 tbsp oil to the skillet. Stir-fry bell peppers and onion for 2–3 minutes until crisp-tender.
Combine and Finish:
Return beef to the skillet and pour in the sauce. Stir-fry for 2–3 minutes until the sauce thickens and beef is fully cooked.
Serve:
Garnish with green onions or sesame seeds if desired. Serve hot over steamed rice or noodles.