Finely chop one medium onion and a handful of fresh parsley. Set aside.

In a large mixing bowl, combine ground beef, ground pork, chopped onion, parsley, breadcrumbs, egg, and mustard. Season with salt and pepper and mix gently for about 3 minutes.

Shape the mixture into oval or round patties, each about 1 inch thick. Aim for about 8-10 patties.

Heat a skillet over medium heat and add enough butter or oil to coat the bottom. Cook the patties for 4-5 minutes on each side until golden brown.

Transfer cooked patties to a plate lined with paper towels to absorb excess oil and let them rest for a few minutes.