Start by thinly slicing the flank steak or sirloin against the grain.
In a medium bowl, combine the sliced beef with 2 tablespoons soy sauce, cornstarch, sesame oil, and black pepper. Toss to coat evenly, then let it marinate for at least 10 minutes while you prepare the rest of the ingredients.
In a small bowl, whisk together soy sauce, oyster sauce, hoisin sauce, brown sugar, grated ginger, minced garlic, red pepper flakes (if using), beef broth, and cornstarch.
Set your Blackstone griddle to medium-high heat and let it preheat for a few minutes. Drizzle 1 tablespoon of vegetable oil over the surface to prevent sticking.
Add the broccoli florets and sliced onions to the griddle. Stir-fry for about 3-4 minutes, tossing occasionally, until the broccoli is bright green and slightly tender but still crisp. Move the vegetables to the side of the griddle.
Add another tablespoon of oil to the center of the griddle. Spread the marinated beef out in a single layer and let it sear for about 2 minutes without touching it. Flip the beef and cook for another 1-2 minutes until beautifully caramelized and browned.
Pour the prepared sauce over the beef and broccoli. Toss everything together, allowing the sauce to coat the ingredients evenly. Let the sauce bubble and thicken for 1-2 minutes.
Garnish with sesame seeds and sliced green onions. Serve hot over steamed jasmine rice, fried rice, or noodles.