Prepare the beef: Slice flank steak against the grain into thin strips. Marinate with 1 tbsp soy sauce and 1 tsp cornstarch for 10 minutes.
Mix the sauce: In a small bowl, whisk together soy sauce, oyster sauce, rice vinegar, beef broth, sugar, and 1 tbsp cornstarch until smooth. Set aside.
Sear the beef: Heat 1 tbsp vegetable oil in a large skillet or wok over high heat. Add beef in a single layer and sear for 2-3 minutes per side until browned. Remove and set aside.
Cook the vegetables: In the same skillet, add remaining 1 tbsp oil. Sauté garlic and ginger for 30 seconds until fragrant. Add bell peppers and onions, stir-fry for 3-4 minutes until crisp-tender.
Combine and simmer: Return beef to the skillet. Pour sauce over beef and vegetables. Stir to coat and simmer for 2-3 minutes until sauce thickens.
Garnish and serve: Sprinkle with sesame seeds and green onions, if using. Serve hot with steamed rice or noodles.