Mix together the ground meat, mustard, mayonnaise, about 1 tsp. of salt and 1/2 tsp. of pepper. Divide the mixture into two portions; form each portion into a patty about 3/4-inch thick.
Heat a skillet that’s wide enough to accommodate both patties at the same time. If not using a non-stick skillet, pour in about a teaspoonful of oil and swirl to cover the entire bottom of the pan. It sounds like too little oil but there’s mayonnaise in the patties and that will provide all the oil that the meat needs during cooking.
Over high heat, cook the patties for four minutes on the first side. Flip, lower the heat to medium and cook for another four minutes on the other side.
Transfer the cooked burgers to a plate; cover loosely with foil (the foil trick does wonders to keep the burgers moist).
Pour off any oil in the skillet but leave whatever bits are stuck at the bottom.
Drop the butter into the skillet. Melt gently, over low heat, scraping the bits left on the bottom of the pan. Turn up the heat, add the mushrooms and cook for about a minute. Pour in the cream. Season with salt and pepper. As soon as the cream simmers, turn off the heat.
Place a burger on a plate and spoon the creamy mushrooms over it. Sprinkle with chopped parsley. Serve the pork burger steaks with creamy mushroom sauce with rice or bread.