Mix together the soy sauce, honey, and vinegar then blend in garlic powder and ginger. Add oil and chopped onion. Trim all excess fat from meat.
Score both sides of flank steak diagonally in a grid like pattern. Place meat in a flat glass dish or casserole and pour the mixture over it; let stand at least 4 hours, preferably overnight. Turn meat occasionally.
Barbeque over a medium high heat for approximately 5 minutes per side for medium-rare steak or until cooked to your done preference. Baste occasionally with the excess marinade, making sure that you don’t have any major flame-ups.
Allow steak to rest for 8-10 mins. Slice flank steak thinly on the diagonal to serve.