Mix all marinade ingredients in a small bowl, making sure sugar is completely dissolved.
Place steak in a large plastic zipper freezer bag; pour in marinade. Seal and turn the bag several times to coat the meat with the marinade.
Place in the fridge and let it sit for 6-8 hours. Remove the meat from the fridge about 30 minutes before cooking so the meat can come to room temperature
Grill over high heat on an outside BBQ for about 8 minutes each side for a 1-2-inch thick steak for rare. About 10 minutes each side for medium and 12 minutes each side for well. These times are just an approximate guide—always use an instant meat thermometer to test meat doneness.
For Dressing: Mix all ingredients together until the sugar dissolves. Refrigerate until ready to use.
Mirin is sweet sake. You can find this in the Asian section of your store. If you can't, it's available from Amazon http://amzn.to/2tnQMkj