Rinse and lightly pat dry the steaks, then transfer to a large Ziploc bag. Set aside for about 30 minutes, to allow the meat to come close to room temperature.

Whisk soy sauce, miso, mirin, brown sugar, olive oil, sesame oil, mustard, garlic, and ginger in a bowl; pour ¾ of the marinade over steaks and toss in the bag to coat. Refrigerate for 15-25 minutes (no longer). Reserve remaining marinade.

Heat grill to high. When hot, remove steaks from the bag and allow excess liquid to drip off. Set on a plate or baking sheet, pat dry with a paper towel, and spray with oil. Place steaks on the hottest part of grill until browned, about 1-1 1/2 minutes.

Once seared and browned, flip to sear the other side for 1 minute; reduce the heat to medium-high, or move the steaks to a cooler section of the grill for 4 minutes or so. Baste the steak with the extra marinade, then flip once more. Grill for 1-2 minutes, then flip and baste again. Flip and brush every few minutes, until deliciously caramelized; just 10-12 minutes for medium-rare. I recommend you use an instant-read thermometer, inserted into the thickest part of the meat, until it reads reads 125°-135° for medium-rare. If the outside of the steaks begin to burn before the inside has come to temperature, move to the cooler section of the grill and continue grilling. Rest steaks 8-10 minutes before slicing and serving hot.