Cut flank steak against the grain at a bias then sprinkle with baking soda and ¼ cup water and mix well. Let this marinate for 10 mins then rinse with water. The baking soda helps the more inexpensive cuts of meatlike flank and skirt get super tender. It’s a restaurant secret.
Next, marinate the beef in the remaining ingredients listed and let it sit for 15-20 mins. In the meantime, blanch your broccoli for 30 seconds in boiling water and prepare your sauce by mixing all the ingredients mentioned under "Sauce".
To make the Beef and Broccoli stir-fry, first fry up your steak in batches with a few tablespoons of oil. It will only take 2-4 mins. DO NOT OVERCOOK! Set aside.
In the same wok in which you seared off the steak, add some oil and garlic cloves, followed by the blanched broccoli and your cooked beef and then your sauce. Let the sauce thicken on high heat. Conversely, you can add your sauce first and let it thicken before adding your veg and meat to avoid overcooking.
Once the sauce is lusciously thick, plate out your dish and serve with white rice. Garnish with sesame seeds.
Notes
You may need additional salt for the marinade. I usually add about a ¼ teaspoon but some people may find the dish too salty for their preference, so I'm leaving it as a suggestion.