Grilled Balsamic & Soy Marinated Flat Iron Steak

1/2 onion, chopped
1 tablespoon dijon mustard
3 cloves garlic, chopped
1 tablespoon rosemary
1/4 cup olive oil
1 teaspoon salt
1/4 cup balsamic vinegar
1/2 teaspoon ground black pepper
1/4 cup soy sauce
1 1/2 pounds flat iron steak

-Whisk all of the ingredients together, except the steak, in a bowl
-Put the flat iron steak in a large resealable bag. Pour the marinade into the bag and coat the steak with the marinade. Squeese excess air from the bag and seal. Marinate in the fridge for at least 30 minutes, up to 6 hours
-Preheat the grill to medium high heat & lightly oil the grate
-Remove the steak from the marinade & shake to remove the excess liquid. Reserve marinade
-Cook the steak until firm, hot in the center, and just turning from pink to grey-4-6 minutes per side. Brush occasionally with the reserved marinade. A meat thermometer inserted should read 150 degrees F
-Move the steak to a cutting board and let it rest for 5 minutes before serving
Serves 4