Freeze steaks for 45 minutes or so until they are firm and partially frozen then remove excess fat and slice very thin; place in a large bowl or resealable plastic bag.
In a small frypan toast sesame seeds over high heat swirling pan being careful not to burn; remove from heat and cool.
Place sesame seeds in a mortar and grind finely but do not grind into a paste. Alternatively you may use a small coffee grinder set on pulse.
Place sesame seeds in a small bowl and mix with miso, soy sauce, sugar and sake (it should have the consistency of runny peanut butter).
Add mixture to meat and work through as a marinade.
Allow meat to marinate for an hour.
Heat a large frypan on high, add a little oil and cook beef without crowding; you may want to cook in batches.
Cook beef about 3-5 minutes or until done to your liking.
Alternatively you may place meat under the grill and cook several minutes or until cooked through.