Using a blender or food processor, puree sake, onions, brown sugar, miso, wine, pepper, and garlic until well combined. Pour mixture into a resealable plastic bag. Place steak in bag, seal, and chill at least 2 hours; flip bag a few times.
Take steak out of bag, saving liquid. Remove excess liquid with paper towels. Heat a cooking spray coated frying pan over medium high heat. Place steak in pan and cook 1 minute per side.
Take steak out of pan and pour saved liquid in. Cook mixture 4 minutes or until sauce becomes thick, stir continually. Place steak back in pan and coat with liquid. Cook another 2 minutes or until preferred degree of doneness. Let steak stand 5 minutes before slicing.