Whisk soy sauce, vinegar, brown sugar, green onions, garlic, and mustard powder together in a bowl; pour into a resealable plastic bag. Add steak, coat with marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator, 3 to 4 hours.

Remove steak from marinade; shake off excess. Discard remaining marinade.

Heat a non-stick skillet over medium-high heat. Add steak; cook until outside is seared, about 1 minute per side. Continue cooking until desired doneness is reached, about 5 minutes more per side. An instant-read thermometer inserted into the center should read 140°F (60°C) for medium.