Place the thinly sliced flank steak in a zip-top bag.
Whisk together the cornstarch and cold water. Pour over the steak, toss to coat, and marinate for 20–30 minutes.
Heat the vegetable oil in a large skillet over medium-high heat.
Add the broccoli, cover, and cook for 3–4 minutes, until slightly tender.
Add the marinated steak. Cook uncovered for another 3–4 minutes, until the beef is cooked through and the broccoli is tender.
In a small bowl, whisk together all of the sauce ingredients.
Pour the sauce over the beef and broccoli. Toss to coat and cook for 1 minute, until heated through.
Remove from the heat and serve warm over cooked rice.
Notes
We used flank steak for this recipe, but you can easily use skirt steak or sirloin steak as well. Refrigerate the leftovers in an airtight container for up to 5 days.