Place the bread and milk in the bowl of a stand mixer fitted with the paddle attachment. Let sit until the bread has absorbed the milk, about 5 minutes.
Meanwhile, heat 1 tablespoon of the butter in a large frying pan over medium heat. Once hot, add the onion and cook until softened, about 4 minutes. Remove from heat and let cool slightly.
Add the cooled onion, beef, pork, egg yolks, allspice, nutmeg, salt, and pepper to the bread and milk. Beat on medium speed until well combined, 1 to 2 minutes.
With a medium spring loaded scoop, shape the mixture into 1 1/2 tablespoon sized balls. Roll the balls between your palms to smooth, wetting your hands if the mixture is sticking.
Wipe the frying pan clean with a paper towel. Heat the remaining 2 tablespoons of butter over medium heat. Add half the meatballs and fry, turning occasionally, until browned on all sides and almost cooked through, 8 to 10 minutes. Transfer to a dish and repeat with the remaining meatballs.
Turn the heat to low and add the flour to the pan drippings. Whisk until lightly browned, about 1 to 2 minutes. Slowly add the beef stock, whisking constantly. Continue to cook and whisk until the mixture thickens and is lump-free, about 5 minutes. Stir in the sour cream and season to taste with salt and pepper. Return the meatballs in the sauce and simmer until cooked through, about 5 to 10 minutes. Sprinkle with parsley before serving.