Place the onion slices in a gallon ziploc bag. Gently press the onions until they release some of their juices. Combine the oil, wine, garlic, juniper berries, thyme, oregano, paprika, cayenne pepper, and black pepper with the onions and mix add to bag, combine.
Rub the Steaks with the Mustard on bothsides
Add the steaks to the bag. Make sure they pork and onions all get well coated with the marinade Refrigerate for at least 24 hours.
Remove pork from refrigeratorand allow to come to room temperature. Wrap the onions separately with foil to create a pouch. Discard marinade.
Preheat grill for medium heat.
Grill the pork steaks and onions until the pork is no longer pink in the center, about 10 minutes per side.