In a medium bowl, whisk together 2 tbsp soy sauce, oyster sauce, hoisin sauce, rice vinegar, 1 tsp sesame oil, cornstarch, garlic, and ginger. Add the thinly sliced beef, toss to coat thoroughly, and let marinate for at least 10-15 minutes while you prep the vegetables.
Prepare all vegetables as listed. Bring a pot of salted water to a boil. Blanch the broccoli florets for 2 minutes, then immediately drain and rinse under cold water to stop the cooking and set the vibrant green color. Pat dry.
Heat a large wok or skillet over high heat. Add 1 tbsp vegetable oil. Once shimmering, add the marinated beef in a single layer (work in batches if needed). Sear without moving for 1 minute, then stir-fry for 1-2 more minutes until just browned but not fully cooked through. Transfer beef to a clean plate.
Add the remaining 1 tbsp oil to the hot wok. Add the sliced onions and the white parts of the green onions. Stir-fry for 2 minutes until slightly softened. Add the bell peppers and stir-fry for another 2 minutes. Finally, add the blanched broccoli and stir-fry for 1 minute until everything is hot and crisp-tender.
Return the beef and any accumulated juices to the wok with the vegetables. Pour in the remaining 1 tbsp soy sauce and the water or broth. Add the green parts of the green onions and the remaining 1 tsp sesame oil. Toss everything together vigorously for 1-2 minutes until the sauce thickens slightly and coats everything evenly.
Divide cooked rice among four shallow bowls. Top generously with the hot beef and vegetable stir-fry. Garnish with sesame seeds and serve immediately.