Place flank steak in a heavy-duty zip-top plastic bag; add Basic Marinade. Seal bag, and chill 8 hours, turning occasionally.
Remove steak from marinade, discarding marinade.
Grill, covered with grill lid, over medium-high heat (350° to 400°) 8 to 10 minutes on each side or to desired degree of doneness. Cut steak diagonally across the grain into thin strips. Serve over salad greens with Horseradish Sauce.