Pour the olive oil and balsamic vinegar together. Add the mustard, garlic, and chopped rosemary slowly.

Whisk the mixture until it looks smooth and thick. Make sure the mustard is fully blended in well.

Place your flat iron steak into the plastic bag. Pour the marinade over the meat very carefully.

Squeeze the extra air out of the bag tightly. Seal it and shake to coat the steak.

Put the bag in the fridge for two hours. This lets the flavors soak deep into the meat.

Preheat your grill to a very high heat setting. Remove the steak from the bag and discard liquid.

Place the steak on the hot grill grates. Grill for five minutes on each side for medium.

Remove the steak and let it rest ten minutes. This keeps the juices inside the meat for later.