place steak, soy sauce, oil, sugar, vinegar, pepper flakes and salt in a large resealable plastic bag. seal tightly and shake back and forth to allow ingredients to blend evenly. refrigerate 24-48 hours, turning occasionally.
preheat grill or broiler. coat cold grill rack or broiler rack with cooking spray and place over grill or broiler pan. remove beef from marinade, reserving marinade. place beef on top of rack and cook 5 minutes, turn and cook 4 minutes longer or until desired doneness. remove from rack and place on cutting board 5 minutes before thinly slicing.
meanwhile, place reserved marinade in a small saucepan. bring to boil over high heat and continue boiling 2 minutes or until reduced to 1/4 cup. watch carefully to not over reduce. remove from heat. place sliced beef on serving platter and drizzle sauce evenly over steak.