Asian Flat Iron Steak

Step 1
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.

Step 2
Remove steak from marinade and discard remaining marinade.

Step 3
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.