Mix together the onion, pork, egg yolk and half the dill in a bowl. Lightly wet your hands, then shape the mixture into 12 balls, each about the size of golf ball.
Heat the oil in a large frying pan. Tip in the meatballs and fry, turning often, for about 12-15 mins until golden all over. Mix together sour cream and remaining dill, then spoon over the meatballs. Serve with mashed potato or tagliatelle.
Minced lamb will work just as well here if you fancy a change from pork.