In a bowl, whisk together all the liquids (reserving 1/4 cup wine for later), garlic, 1 tbsp rosemary (reserving 1 tbsp for later), orange peel, salt and pepper. Place the marinade in sealable bag, and then insert the flat iron steak. Tie the bag closed, and refrigerate for 10 hours, or overnight.
Pre-heat grill to medium-high heat, cook the steak 3-4 minutes per side until medium rare. Remove the steak from the grill and let rest for for 3-5 minutes.
While the steak rests, pour the marinade and remaining wine into a skillet. Bring to a boil and reduce the liquid by half. Stir in remaining rosemary.
Slice the flat iron against the grain and top with sauce reduction.