Step 1: Prepare the beef. Thinly slice the flank steak against the grain into bite-sized pieces. In a medium bowl, combine the beef with 1 tbsp soy sauce, 1 tsp baking soda, 1 tbsp cornstarch, and 1 tbsp vegetable oil. Mix well to coat and let marinate for at least 15 minutes at room temperature.
Step 2: Blanch the broccoli. Bring a pot of lightly salted water to a boil. Add the broccoli florets and cook for 2-3 minutes until bright green and crisp-tender. Immediately drain and transfer to an ice bath to stop the cooking. Drain well and set aside.
Step 3: Make the sauce. In a small bowl, whisk together the beef broth, 1/4 cup soy sauce, oyster sauce, brown sugar, and sesame oil. In a separate tiny bowl, combine 1 tbsp cornstarch with 2 tbsp cold water to create a smooth slurry. Set both aside.
Step 4: Stir-fry the beef. Heat 1 tbsp vegetable oil in a large wok or skillet over high heat until shimmering. Add the marinated beef in a single layer (cook in batches if necessary to avoid overcrowding) and stir-fry for 2-3 minutes until browned and cooked through. Remove beef from the wok and set aside.
Step 5: Sauté aromatics. Add the remaining 1 tbsp vegetable oil to the wok. Add the grated ginger and minced garlic and stir-fry for 30 seconds until fragrant.
Step 6: Combine and thicken. Return the cooked beef to the wok. Add the blanched broccoli. Give the sauce mixture a quick stir and pour it over the beef and broccoli. Bring to a simmer.
Step 7: Add cornstarch slurry. Re-whisk the cornstarch slurry and slowly pour it into the simmering sauce, stirring constantly until the sauce thickens to your desired consistency.
Step 8: Serve immediately. Serve the Homemade Chinese Beef and Broccoli hot over steamed rice, garnished with sesame seeds if desired.