Marinate the Beef: In a bowl, mix soy sauce, oyster sauce (if using), and cornstarch. Add the sliced beef and coat well. Let marinate for at least 15 minutes.
Stir-Fry the Vegetables: Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the onion and bell peppers and stir-fry for 3–4 minutes until softened. Add the garlic and ginger, and cook for 1 minute. Remove the vegetables from the skillet and set aside.
Cook the Beef: Heat the remaining 1 tablespoon of oil in the same skillet over high heat. Add the marinated beef in a single layer and sear for 2–3 minutes on each side until browned. Return the vegetables to the skillet and toss everything together. Season with black pepper and red pepper flakes (if using). Cook for an additional 1–2 minutes.
Assemble the Rice Bowl: Toss the cooked rice with soy sauce and sesame oil (if using). Divide the rice among serving bowls and top with the beef and vegetable mixture.
Garnish: Sprinkle the bowls with sesame seeds and sliced green onions.
Serve: Serve hot and enjoy!