Combine all the marinade ingredients in a saucepan placed over a medium-high heat. Season to taste and bring to a boil; then reduce the heat and let simmer for 5 minutes.
Remove the marinade from the heat and let it cool.
Place the steaks in a marinating container. Cover with the marinade and refrigerate for 8 to 12 hours.
If you’re cooking your steaks on the grill, preheat the grill to a medium-high heat.
Place a skillet over a low heat and melt some cooking fat. Add the onions and cook, stirring occasionally, for 30-40 minutes until the onions are caramelized. Season to taste.
Remove the steaks from the marinade and cook on the preheated grill or skillet for 3 to 4 minutes per side, or to your desired doneness.
Serve the steaks with hot caramelized onions on top.