To make the marinade: combine the miso, sake, mirin and sugar in a saucepan and bring to a boil over medium heat
Reduce the heat and continue to stir over a low heat for about 20 minutes, until the sauce thickens, being careful not to let it burn
To prepare the steaks: spread 2 Tbsp of the marinade on each side of the steaks and cover in plastic wrap. Leave the meat in the refrigerator for no less than 12 hours or overnight
When ready to cook, remove the steaks from the refrigerator and allow to come to room temperature. Remove the excess marinade from the steaks using a spatula. Preheat a grill and sear the steaks quickly on both sides. Turn down the heat and cook according to personal preference. The steaks can be pan-fried too, if preferred
Let the steak rest for a minute and then slice the steaks and serve with grilled vegetables and wasabi