Marinate the beef: Cut the flank steak across the grain into slices 1/4 inch thick. In a shallow dish large enough to hold the meat, stir together the sake, soy sauce, vinegar, honey, sesame oil, ginger, garlic, red pepper flakes and coriander. Add the flank steak and stir to coat thoroughly. Cover and let stand for at least 30 minutes to 2 hrs.