1. Soak skirt steak in cold water for 30 minutes, then rinse repeatedly under cold water.
2. In medium bowl, whisk soy sauce, garlic, ginger, vinegar, sesame oil, sugar, and crushed red pepper until sugar dissolves. Reserve two tablespoons marinade and set aside. Transfer remaining marinade to large zip-top plastic bag.
3. Thinly slice three green onions and set aside. Cut remaining green onions into long pieces. Add steak and long green onions to bag, turning to coat. Close bag and refrigerate two hours or overnight, turning over several times.
4. Preheat gas grill to 375 degrees Fahrenheit. Remove steak and green onions from marinade, grease grill, and place steak and onions on hot grill grate. Discard marinade in bag. Cover grill and cook steak and onions for four to five minutes. Turn and cook on the other side for another four to six minutes for medium-rare (about 135 degrees Fahrenheit) or until desired doneness. Remove steak and onions from grill. Let steak stand 10 minutes to set juices for easier slicing.
5. In small bowl, combine two tablespoons hot water with reserved marinade. Serve steak with grilled spring onions, reserved sliced spring onions, and parsley.