Mix together the onion, pork, egg yolk, salt, pepper, lemon juice 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.