- 1/2 cup reduced-sodium soy sauce
- 1/4 cup rice wine vinegar
- 1/4 cup brown sugar
- 1/4 cup sliced green onions
- 1 tablespoon minced garlic
- 1 tablespoon mustard powder
- 2 pounds flat iron steak
- 2 tablespoons sesame seeds (optional)
Whisk soy sauce, vinegar, brown sugar, green onions, garlic, and mustard powder together in a bowl; pour into a resealable plastic bag. Add the steak, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator, 3 to 4 hours.
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Remove steak from marinade and discard remaining marinade.
Heat a non-stick skillet over medium-high heat; cook steak just until outside is browned and seared, about 1 minute per side. Continue to cook until desired doneness is reached, about 5 minutes more per side. An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C). Remove skillet from heat and allow steak to rest before cutting, 3 to 5 minutes. Sprinkle sesame seeds over steak.