Pour soy sauce, water, and vinegar into a large, non-metallic bowl. Whisk
in brown sugar, white sugar, pepper, sesame oil, garlic, and onion until the
sugars have dissolved. Submerge the steaks in this marinade, and cover
with plastic wrap. Refrigerate 7-12 hours; the longer, the better.
Preheat an outdoor grill for medium-high heat.
Remove steaks from the marinade, shake off excess, and discard the
marinade. Cook on preheated grill until the meat is no longer pink, 5-7 min.