Preheat the slow cooker if necessary; see the manufacturer's instructions. Finely chop the onion and bread in a food processor or liquidizer. Add the minced meats, mixed spice, egg yolk and a little seasoning and mix together.
Divide the meat mixture into 24 and roll into balls with wetted hands. Heat the oil in a frying pan, add the meatballs and fry over a medium heat, turning until evenly browned but not cooked through. Drain and transfer to the slow cooker pot.
Make the sauce. Add the butter and onion to the cleaned frying pan. Fry, stirring, for 5 minutes or until the onion is softened and just beginning to turn golden. Stir in the flour, then gradually mix in the stock and bring to the boil, stirring. Season and pour the sauce over the meatballs. Cover with the lid and cook on low for 6–8 hours.
Stir the chopped dill into the sauce and serve the meatballs with mashed potato sprinkled with a little extra chopped dill to garnish.