In a medium bowl, combine the sliced flank steak, 2 tablespoons of cornstarch, soy sauce, and sesame oil. Toss to coat the beef evenly. Let it marinate for at least 10 minutes.

In a small bowl, whisk together the beef broth, soy sauce, brown sugar, and oyster sauce (if using). Set aside.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated beef in a single layer (cook in batches if necessary to avoid overcrowding). Sear for 1-2 minutes per side until browned but not fully cooked through. Remove beef from the skillet and set aside.

Add the remaining 1 tablespoon of vegetable oil to the skillet. Add the minced ginger and garlic and stir-fry for about 30 seconds until fragrant.

Add the broccoli florets to the skillet and stir-fry for 2-3 minutes. Add about 1/4 cup of water, cover, and steam for 3-5 minutes, or until the broccoli is crisp-tender.

Return the seared beef to the skillet. Pour the prepared sauce mixture over the beef and broccoli. Bring to a simmer.

In a small cup, whisk together the 1 tablespoon of cornstarch with 2 tablespoons of water to create a slurry. Pour the slurry into the simmering sauce, stirring constantly, until the sauce thickens to your desired consistency.

Stir in the sliced green onions. Serve immediately over rice, garnished with sesame seeds if desired.