In a medium bowl, combine the thinly sliced flank steak with 1 tbsp soy sauce, 1 tsp rice vinegar, 1 tbsp cornstarch, 1 tsp sesame oil, and 1/4 tsp baking soda. Mix well to coat the beef evenly. Let it marinate at room temperature for at least 15-20 minutes while you prepare other ingredients.

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

In a small bowl, whisk together the chicken broth, 2 tbsp soy sauce, oyster sauce (if using), brown sugar, 1 tsp rice vinegar, 1/2 tsp sesame oil, and 1 tbsp cornstarch until smooth and no lumps remain. Set aside.

Heat 1 tablespoon of vegetable oil in a large wok or skillet over high heat until shimmering. Add half of the marinated beef in a single layer (do not overcrowd the pan). Stir-fry for 2-3 minutes until browned and cooked through. Remove the cooked beef to a clean plate. Add the remaining 1 tablespoon of oil and cook the rest of the beef, then remove and set aside with the first batch.

Reduce heat to medium-high. Add the minced garlic and grated ginger to the wok (add a tiny bit more oil if the pan is dry). Stir-fry for 30 seconds until fragrant.

Add the blanched broccoli to the wok. Stir-fry for 1-2 minutes to heat through and coat with the aromatics.

Give the prepared sauce a quick re-whisk. Pour the sauce into the wok. Bring to a simmer, stirring constantly, until the sauce thickens to your desired consistency, about 1-2 minutes.

Return the cooked beef to the wok. Toss everything gently to coat the beef and broccoli evenly with the thickened sauce.

Serve immediately over hot steamed white rice. Garnish with sesame seeds or red pepper flakes if desired.