In a medium bowl, combine the thinly sliced flank steak with 1 tbsp soy sauce, 1 tbsp cornstarch, 1 tsp sesame oil, and 1/2 tsp baking soda (if using). Mix well and let marinate at room temperature for at least 15 minutes, or refrigerate for up to 30 minutes.

Prepare the sauce: In a small bowl, whisk together the beef broth, 1/4 cup soy sauce, oyster sauce, and brown sugar. Set aside.

In another small bowl, whisk together 1 tsp cornstarch and 2 tbsp water to create a slurry. Set aside.

Bring a pot of salted water to a boil. Add the broccoli florets and blanch for 2-3 minutes until bright green and tender-crisp. Drain immediately and set aside.

Heat 1 tbsp vegetable oil in a large skillet or wok over high heat. Add the marinated beef in a single layer (cook in batches if necessary to avoid overcrowding). Cook for 1-2 minutes per side until browned, then remove from the skillet and set aside.

Add the remaining 1 tbsp vegetable oil to the skillet. Add the minced garlic and grated ginger, and stir-fry for 30 seconds until fragrant.

Pour in the prepared sauce mixture. Bring to a simmer, then stir in the cornstarch slurry. Cook, stirring constantly, until the sauce thickens.

Return the cooked beef and blanched broccoli to the skillet. Toss everything together to coat evenly with the sauce.

Serve immediately over hot cooked rice.