01 - Combine sliced beef with soy sauce, oyster sauce, cornstarch, and sesame oil in a bowl. Mix thoroughly and let marinate for 10 minutes.
02 - Whisk together all sauce ingredients in a separate bowl until smooth and set aside.
03 - Bring salted water to a boil. Blanch broccoli florets for 1 to 2 minutes until bright green and tender. Drain and set aside.
04 - Heat 1 tablespoon vegetable oil in a wok or large skillet over high heat. Stir fry marinated beef for 2 to 3 minutes until browned but not fully cooked. Remove beef and set aside.
05 - Add remaining vegetable oil to the pan. Sauté garlic, ginger, and red bell pepper (if using) for 1 minute until fragrant.
06 - Return beef and broccoli to the pan. Stir sauce to recombine, then pour it over. Cook while stirring for 2 to 3 minutes until sauce thickens and coats ingredients evenly.
07 - Serve immediately, ideally over steamed rice.