Mix the soy sauce, sake, mirin and sugar in a bowl big enough for the steaks. Add the steaks and marinate for at least 15 minutes or up to an hour.
Cook the rice according to the pack instructions (see note above). Once cooked, add the sesame oil, and some sea salt and keep warm.
Heat a griddle pan or heavy based frying pan on a high heat. Remove the steaks from the marinade and pat dry. Pour the marinade into a small pan and set aside.
Lightly brush each steak with vegetable oil and season with salt, place on the griddle pan and sear for 2-3 minutes on each side. Allow to rest for at least 5 minutes.
While the steak is resting, bring the reserved marinade to a boil and bubble for 1 minute. remove from the heat and whisk in the butter for a lovely shiny sauce.
Serve the beef thinly sliced with some sauce drizzled over the top and the rice in a bowl sprinkled with the furikake and chilli flakes for everyone to help themselves