Place flank steak in a gallon ziploc bag.
Mix all marinade ingredients then pour into bag with steak, seal bag. Marinate in the refrigerator for at least 2 hours, 3-4 hours for best results.
Heat cast iron pan on stove over medium/high heat for 3-4 minutes.
Take flank steak out of bag and allow any excess marinade to drip off. Place steak into hot pan and cook each side for approximately 4 minutes.
4 minutes each side will typically make a steak on the rare side (depending on thickness of flank steak). 130º-135º internal temp will be medium rare. Adjust cooking time according to preferences.
Remove flank steak from pan and transfer to a cutting board. Season each side with sea salt and allow to rest for 5 minutes before slicing.
Slice against the grain and serve.