Cut the steak lengthwise in half, then thinly slice against the grain. Put in a bowl.

Toast the sesame seeds in a dry skillet, Reserve 3 tablespoons.

Crush remaining seeds using a mortar and pestle. Add to the beef.

Add the soy, oil, brown sugar, garlic, pepper, 3 of the 4 green onions and red pepper flakes to the beef, stir and allow to marinate, refrigerated, for 1-24 hours.

Heat a large cast iron frying pan for several minutes on high. Add half of the beef and cook for 3-5 minutes, transfer to a warmed serving bowl and cook the remaining beef.

Sprinkle with reserved toasted sesame seeds and green onions before serving with steamed rice.