Wipe mushrooms clean with a moist towel and cut shallow slits on top of the mushroom caps (be careful you don't cut all the way through). This helps the mushroom absorb the marinade.
Add the remaining ingredients (except the onions) to a gallon-size Ziploc bag and squish the bag to mix the ingredients. Add the mushrooms and seal the bag. Let marinade for about an hour.
Heat cast iron skillet to medium-high. When the pan is hot, add mushrooms. Add sliced onion to the remaining marinade and add to the pan as well. Depending on the thickness of the mushrooms, you may need to add a lid to help them cook through.
