Place all the ground meat, breadcrumbs, Parmesan and parsley into a large bowl and, using your hands, mix thoroughly. (You could just use beef mince if you prefer.)
Add the onion, olive oil, egg yolk and a generous pinch of salt and freshly ground black pepper and mix again.
Using clean hands divide the meatball mixture into small pieces and roll into balls. The size of a table-tennis ball is a good guide.