Slice the flank steak into very thin strips. For easier slicing, place the steak in the freezer for 10–15 minutes before cutting. Season with black pepper.
If desired, blanch the broccoli in boiling water for 2–3 minutes until bright green. Drain, rinse under cold water, and pat dry.
In a small bowl, whisk together the soy sauce, water, brown sugar, garlic, ginger, and cornstarch until smooth.
Heat half of the sesame oil in a large skillet or wok over high heat.
Add the beef in a single layer and cook, stirring frequently, until just cooked through. Remove the beef from the skillet and set aside.
Add the remaining sesame oil to the skillet. Stir-fry the broccoli for 1–2 minutes. If you skipped blanching, cook for 4–5 minutes, adding a splash of water if needed.
Return the beef to the skillet and pour in the prepared sauce.
Stir everything together and bring the sauce to a gentle boil. Cook for about 1 minute until the sauce thickens and evenly coats the beef and broccoli.
Serve the Healthy Beef and Broccoli immediately with your favorite side dish