Rub garlic and onions and parsley into meat, then marinade in the oil/vinegar mix (mostly oil). Cover top with more garlic, onions and parsley and let sit covered for several hours, turning occasionally.

Strip the onions off the meat and saute in more balsamic vinegar until onions sweet and translucent. Hold as sauce for meat.

Broil to preferred doneness, but keep watch because the meat is thin.