Step 1: Prepare the Beef. In a medium bowl, combine the thinly sliced beef with 1 tablespoon soy sauce, 1 teaspoon cornstarch, and 1/2 teaspoon baking soda (if using). Mix well to coat every piece. Let it marinate for at least 15 minutes while you prepare the other ingredients.
Step 2: Whisk Together the Sauce. In a small bowl, whisk together the beef broth, 1/4 cup soy sauce, oyster sauce, brown sugar, rice vinegar, sesame oil, and half of the minced garlic and ginger. Set aside. In another small bowl, create your cornstarch slurry by mixing 1 tablespoon cornstarch with 2 tablespoons cold water until smooth; set aside.
Step 3: Blanch the Broccoli. Bring a medium pot of water to a boil. Add the broccoli florets and blanch for 1-2 minutes until bright green and crisp-tender. Immediately drain and plunge into an ice bath to stop the cooking and preserve their vibrant color. Drain very well and set aside.
Step 4: Cook the Beef. Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat until shimmering. Add the marinated beef in a single layer (cook in batches if necessary to avoid overcrowding). Cook for 2-3 minutes per side until browned and cooked through. Remove the beef from the pan and set aside.
Step 5: Sauté Aromatics and Finish the Sauce. Add the remaining 1 tablespoon of vegetable oil to the pan. Add the remaining minced garlic and ginger and sauté for 30 seconds until fragrant. Pour in the prepared sauce mixture and bring it to a gentle simmer.
Step 6: Thicken and Combine. Once the sauce is simmering, slowly whisk in the cornstarch slurry until the sauce thickens to your desired consistency – it should be glossy and able to coat the back of a spoon. Return the cooked beef and blanched broccoli to the pan with the sauce. Toss everything gently to coat evenly. Cook for another 1-2 minutes, just long enough to heat through.
Step 7: Serve. Serve immediately over steamed white or brown rice. Garnish with sesame seeds and sliced green onions, if desired.