In a bowl whisk together garlic, extra virgin olive oil, balsamic, 1/4 cup red wine, 1 tablespoon chopped rosemary, salt and pepper.
Place flat iron steak in a resealable plastic bag. Pour marinade over the top. Shake to coat steak in marinade. Refrigerate for 8 hours or overnight.
When you're ready to cook, preheat your grill to medium-high heat.
Place the steak on the grill and reserve the marinade. Cook steak 3 - 4 minutes per-side for medium rare. Remove steak from grill and let rest for 10 minutes. Remember the steak will continue to cook a bit while resting.
Meanwhile, pour marinade into a skillet over medium heat. Pour in remaining 1/4 cup of red wine. Bring to a boil and allow to cook until it reduces by half, 5 - 10 minutes. Stir in remaining tablespoon of fresh rosemary. Remove from heat.
After steak has rested, slice it against the grain into 1/2 inch slices. Spoon sauce over the top and serve.