In a bowl, whisk together soy sauce, olive oil, balsamic vinegar, garlic, black pepper, and rosemary.

Place the flat iron steak in a resealable plastic bag and pour the marinade over it. Seal the bag and refrigerate for at least 1 hour, or up to 8 hours.

Preheat the grill to high heat.

Remove the steak from the marinade and discard the marinade. Let the steak sit at room temperature for about 15 minutes.

Place the steak on the grill and cook for about 4-5 minutes on each side for medium-rare, or until desired doneness is reached.

Use a meat thermometer to check the internal temperature; it should read 130°F for medium-rare.

Remove the steak from the grill and let it rest for 5 minutes before slicing.

Slice the steak against the grain and serve with your favorite sides.