In a medium skillet over medium-high heat, saute 1 cup of the onion in 1 1/2 tablespoons of the butter until soft and light golden brown, 4 to 6 minutes. Set aside to cool, then add the onion to a medium mixing bowl along with the ground chuck, ground pork, 1 1/2 teaspoons of the salt, 1/2 teaspoon of the pepper, the allspice, and the nutmeg.

In a small mixing bowl, combine the bread crumbs with the milk and 1/4 cup of the cream, and allow to sit until the bread crumbs are soft, about 5 minutes. Add the bread-crumb mixture and the beaten egg to the meat mixture and, using clean hands, mix well to thoroughly combine.

Using a tablespoon measure as a guide, scoop the meat mixture into small meatball portions, about 1 heaping tablespoon each. Using lightly damp clean hands, roll the meatballs until they are smooth and round. Transfer to a large plate or baking dish while you form the remaining meatballs.

In a medium skillet, heat 1/2 tablespoon of the remaining butter until foamy. Add one-third of the meatballs and cook, turning frequently, until the meatballs are browned on all sides, 6 to 8 minutes total. Using tongs or a slotted spoon, transfer the cooked meatballs to a Dutch oven or heavy large saucepan and repeat with the remaining meatballs, adding another 1/2 tablespoon of butter to the skillet before each batch.

Add the remaining 1 1/2 tablespoons of butter and the remaining 1/2 cup onion to the skillet and cook until the onion is soft, about 4 minutes. Add the flour and cook, stirring constantly, for 2 minutes. Whisk in the beef broth little by little and cook until the sauce is smooth and thick, about 4 minutes. Transfer the sauce to the Dutch oven or large saucepan and add the remaining 1/4 teaspoon salt, a pinch of black pepper, the remaining heavy cream, and the currant jelly. Bring the contents of the Dutch oven or large saucepan to a gentle boil over medium-high heat, then reduce the heat so that the sauce just simmers. Stir very gently to combine ingredients once the meatballs have cooked a bit; be careful not to break up the meatballs. Cook, uncovered, for 45 minutes to 1 hour, until the meatballs are very tender and the sauce is thick and flavorful. Serve with boiled new potatoes, cucumber salad, and lingonberries for a main course or in a shallow chafing dish with toothpicks for hors d'oeuvres.