1) Preheat the oven to 170C/Gas mark 3.
2) Melt 15g of the butter in a large saute pan over medium heat. Add the onion and cook until translucent. Remove the onion to a plate and cool. Wipe out the pan with a kitchen towel.
3) In a large mixing bowl, combine the ground meats, egg yolks, double cream and onion. Add salt, pepper and allspice. Squeeze excess milk from the bread and add it to the bowl. Mix gently with your hands or wooden spoon.
4) With a bowl of cold water standing by, form meatballs about 2.5cm across. Dip your fingers in cold water, from time to time, to keep the meat from sticking to your hands.
5) Melt the remaining butter in the saute pan and once the foam has subsided add a layer of meatballs. Do not overcrowd the pan. Shake the pan once in awhile, to keep the meatballs round. Brown all of the meatballs, in batches, removing them to a large oven safe dish when cooked. Put the dish in the oven for about 20 minutes, being careful not to overcook them.
Cook's Note: you may need to wipe out the pan between batches as it can get kind of gnarly.
6) To make the sauce: Pour off most of the fat from the skillet and return it to the heat. Add 30g of butter and swirl it around to coat the pan. Sprinkle in the flour, and stir with a wooden spoon or whisk to dissolve the flour into the fat. Pour in the chicken stock, and stir to loosen the bits from the bottom of the pan. Simmer, stirring, until the liquid is reduced and the mixture starts to thicken to a sauce. Season with salt and pepper, to taste. Lower the heat and stir in the cream and the black currant jam. Add the meatballs to the sauce. Simmer until the sauce thickens slightly and the meatballs are heated through, 15 to 20 minutes. Sprinkle with chopped parsley.
7) To serve, spoon a couple of meatballs onto each plate along with a spoonful of sauce, and serve with the lingonberry jam on the side.