Combine all ingredients in a glass or ceramic dish. Remove flank steak from package and give it a quick rinse. Coat both sides of the meat with marinade. Refrigerate for at least 3 to 6 hours.
Remove the steak from the marinade and place on a plate while you heat up the grill pan. Make sure to turn on your overhead exhaust fan as the cooking process can be quite smoky. When the pan is extremely hot, place the steak on a slight angle as this makes for more appealing grill marks. Cook without touching for about 1 ½ to 2 minutes. Rotate the meat about 45 degrees, keeping it on the same side, and cook for another 1 ½ minutes.
Now flip it to the other side. Cook using the same method on the other side, cooking for 1 1/2 minutes, then rotating it and finishing the cooking process. I cook the flank streak to medium rare, which usually takes about 5 to 7 minutes total cooking time. Remove the meat to a cutting board and let it rest for a few minutes before slicing.
NOTES: I did not add the sesame oil and we grilled instead of cooking it indoors. We did increase the cooking time by a few minutes, but it was outdoors in the winter; I'm not sure you'd have the same issues with the same recipe grilled in warm weather. The keys to making this tough cut of meat pretty delicious are (1) marinading and (2) thin-ish cuts across the grain of the meat.