In a medium bowl, whisk together the soy sauce, mirin, ginger, garlic, sugar, and sesame oil. Place the steak in a large casserole pan or Ziploc bag. Pour the marinade over the steak. Cover and refrigerate for about 2 hours, but no more than 6.
Remove the steak from fridge and bring to room temperature.
Heat grill or grill pan to high.
Pat the meat dry with a paper towel, then season with salt and pepper. Place on a lightly oiled hot grill. Cook for four minutes on each side for rare. Remove to a cutting board and let rest 5 to 10 minutes.