Blanch the broccoli. Cut the broccoli into bite-size florets. Add to boiling water and blanch for 2 minutes. Drain and set aside.
Make the sauce. Whisk together the beef broth, soy sauce, sesame oil, brown sugar and white pepper. Add the cornstarch and whisk until completely dissolved
Slice the beef. Look for the direction of the muscle fibers running through the flank steak. Turn your knife so you're cutting across the grain, then slice the beef thinly.
Velvet the beef. Add the salt, white pepper and baking soda to the beef and mix thoroughly. Add the water and massage it into the beef until absorbed. Add the cornstarch and mix again. Finally, add the oil and toss until evenly coated. Let the beef rest for 20 minutes.
Cook the beef. Heat a wok over medium-high heat until hot. Add enough neutral oil to lightly coat the cooking surface. Add the beef in a single layer. Let it sear undisturbed for 45 seconds, then stir-fry until browned. Remove and set aside.
Cook the aromatics. Reduce heat to medium. Add the sliced onions to the same wok and cook for 1-2 minutes. Add the garlic and ginger and stir-fry for 30-45 seconds, just until fragrant.
Add the sauce. Give the sauce a good stir because cornstarch settles while it sits. Pour it into the wok and bring to a simmer, stirring as it thickens. Reduce heat to low.
Bring everything together. Return the beef to the wok, followed by the broccoli. Toss gently until everything is well coated and heated through. Garnish with crushed red pepper flakes (optional), and serve immediately over white rice.
