Season the steaks and place in a pan, let them rest in the fridge for at least 2 hours to fully soak the seasoning in.
When ready to grill, remove steaks and let them rest at room temperature for about 30 minutes before grilling.
Preheat the grill to 450°F.
In a grill-safe cast iron skillet, add the balsamic vinegar and olive oil, and swirl to combine.
Add in the mushrooms, onion, garlic, and salt. Set aside.
Place the steaks on the preheated grill, grilling on each side for about 7 minutes or until your desired doneness.
While the steaks are grilling, place the cast iron skillet onto the grill grates and let it cook alongside the steaks, stir or mix after about 7 minutes when flipping the steaks. The onions should be translucent and the mushrooms should be soft when ready to serve.
Carefully remove the steaks and the cast iron skillet. Let steaks rest about 5 minutes before serving.
Top the steaks with the mushroom and onion mixture and serve.
Notes
Average Ribeye Steaks are 12-16oz. We cut ours in half and used 2 ribeyes to serve 4 people. The nutritional information is based on eating 1/2 of a ribeye so about 6-8oz.