Soak the bread crumbs in the milk while you prepare the other ingredients.
Put the beef and pork into a large bowl. Add the onion, butter, eggs, and parsley and season well with salt and pepper. Put the soaked bread crumbs into the bowl and mix it all together well with your (clean) hands. Form the mixture into meatballs about the size of ping pong balls.
Heat about 1/4-inch of oil in a skillet over medium heat. Fry the meatballs until they are browned on all sides and cooked through, about 15 minutes.