In a medium bowl, combine the low sodium soy sauce, water, ground white pepper, brown sugar, minced garlic, and grated ginger. Mix well until the brown sugar is dissolved. This will be the marinade for your beef.
Add the thinly sliced flank steak to the marinade. Let it sit for about 15-30 minutes at room temperature.
While the beef is marinating, bring a pot of salted water to a boil. Add the broccoli florets and blanch for 2-3 minutes until they are bright green and tender-crisp. Drain and rinse under cold water to stop the cooking process. Set aside.
If you prefer a thicker sauce, mix the tablespoon of cornstarch with a little water in a small bowl until smooth. Set this aside; you’ll add it later.
Heat the sesame oil in a wok or large skillet over high heat. Once hot, add the marinated beef in a single layer. Cook for 2-3 minutes until browned.
Add the sliced yellow onion to the beef. Stir-fry for another 2 minutes until the onion begins to soften.
Add the blanched broccoli to the wok and stir everything together. Pour in the marinade and bring it to a simmer. If using cornstarch, add it now and stir to combine until the sauce thickens, about 2 minutes.
Remove the wok from heat. Serve the beef and broccoli over cauliflower rice or white rice. Garnish with toasted sesame seeds and sliced scallions.
