Stir all of the above ingredients together, you can cut the onions into rings or add them in small cubes. The pork steaks are seasoned on both sides with salt and pepper and then coated with mustard. Put some of the grill marinade in an appropriately sized container with a lid, put meat slices on top, then marinade again, add meat slices - so layer everything in the container, with the grill marinade covering the meat at the end. If the marinade is not long enough, simply add beer to it. Put the lid on and put it in the fridge for at least 12 hours.