Put flat iron steak in a freezer bag, pour marinade over it, seal bag, and freeze. The night before you plan to serve this, take bag from the freezer and thaw in refrigerator. (Or marinate the steak for several hours in the fridge, without freezing at all.).
To prepare for serving, remove beef from marinade; reserve marinade.Grill steak for 8 to 10 minutes per side; or broil 6 minutes on one side and 4 on the other.
Put reserved marinade in sauce pan. Heat on medium; let this gently boil for at least 10 minutes or until reduced by about a third.
Let steak rest for 10 minutes. Cut steak across grain at a slanted angle into thin slices.