Stir soy sauce, sugar, scallions, sesame oil, sesame seeds, garlic and ginger together in bowl.
Fold the flank steak into thirds. Place in a large resealable plastic food storage bag. Pour the soy sauce and brown sugar mixture into the bag. Seal bag and turn over to cover steak.
Place in a second bag and label. Freeze for up to 2 months.
Thaw bag in refrigerator for 2 days.
Heat grill or broiler. Remove steak from bag. Place marinade in small saucepan and bring to a boil. Reduce heat and simmer 5 minutes.
Grill or broil steak for 5 minutes per side or until meat registers 135 degree F on an instant-read thermometer for medium-rare. Allow meat to rest for 5 minutes before slicing.
To serve, thinly slice meat against the grain and serve with marinade.
