In a bowl, whisk together all of the liquids (reserving 1/4 cup wine), garlic, 1 tablespoon rosemary, orange peel, salt and pepper. Place the marinade in a zip-top bag and insert the flat iron steak. Seal and place in the refrigerator for 10 hours, or overnight.
On a medium-high grill, cook the steak 3-4 minutes per side (flipping once) until medium rare. Let the steak rest off the heat.
While the steak cooks, pour the marinade into the skillet with the remaining wine and beef stock. Bring to a boil and reduce by half. Stir in the remaining rosemary.
Slice the flat iron against the grain and top with sauce.