Step 1: Slice flank steak very thinly against the grain into bite-sized pieces. Place beef in a medium bowl.
Step 2: Add 1 tablespoon of soy sauce, baking soda, 1 tablespoon of cornstarch, and sesame oil to the beef. Mix well to coat. Marinate for at least 15-20 minutes at room temperature.
Step 3: Cut broccoli into small florets. Bring a small pot of salted water to a boil. Blanch broccoli for 1-2 minutes until vibrant green and tender-crisp. Drain and immediately transfer to an ice bath to stop cooking. Drain again thoroughly.
Step 4: In a small bowl, whisk together the remaining 3 tablespoons of soy sauce, oyster sauce, rice vinegar, sugar, white pepper, and beef broth. Set aside.
Step 5: In an even smaller bowl, whisk together the remaining 1 tablespoon of cornstarch with 2 tablespoons of cold water to create a slurry. Set aside.
Step 6: Mince garlic and grate fresh ginger. Have all ingredients prepped and ready by your stovetop.
Step 7: Heat 1 tablespoon of high-heat cooking oil in a large wok or large non-stick skillet over high heat until shimmering.
Step 8: Add about half of the marinated beef to the hot wok in a single layer. Sear for 1-2 minutes per side until browned. Remove cooked beef to a clean plate and repeat with remaining beef, adding a little more oil if needed.
Step 9: Add another teaspoon of oil to the wok if necessary. Add minced garlic and grated ginger. Sauté for about 30 seconds until fragrant, being careful not to burn.
Step 10: Add the blanched broccoli florets to the wok. Stir-fry for 1-2 minutes, tossing to coat.
Step 11: Give the prepared stir-fry sauce a quick whisk and pour it into the wok. Bring to a simmer, stirring constantly.
Step 12: Give the cornstarch slurry another quick stir and slowly pour it into the simmering sauce, stirring continuously. The sauce will thicken almost immediately. Continue stirring until it reaches your desired consistency (about 30-60 seconds).
Step 13: Return all the cooked beef to the wok. Toss everything together to ensure the beef and broccoli are thoroughly coated in the sauce. Cook for another minute to heat through.
Step 14: Taste and adjust seasonings if needed. Serve immediately over steamed rice or noodles, garnished with sesame seeds or green onions if desired.