In a medium skillet over medium-high heat, sauté 1 cup of the onions in 1 ½ tablespoons of the butter until soft and light golden brown, about 4 to 6 minutes.  Set aside to cool, then add the onions to a medium mixing bowl along with the ground chuck, ground pork, 1 ½ teaspoons of the salt, ½ teaspoon of the pepper, the allspice and nutmeg.  

In a small bowl combine the breadcrumbs with the milk and ¼ cup of the cream and allow to sit until breadcrumbs are soft, about 5 minutes.  Add the breadcrumb mixture and the egg to the meat mixture, and, using your hands, mix well to thoroughly combine.

Using a tablespoon measure as a guide, scoop the meat mixture into meatball portions, about 2 tablespoons of meat each.  Using lightly damp 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 ½ 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.  Using tongs or a slotted spoon, transfer the meatballs to a Dutch oven or other heavy large saucepan and repeat with the remaining meatballs, adding an additional ½ tablespoon of butter to the skillet before each addition.  

Add the remaining 1 ½ tablespoons of butter and the remaining ½ cup of chopped purple onions to the skillet and cook until the onions are 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 and add the remaining ¼ teaspoon salt, a pinch of black pepper, the remaining ¾ cup heavy cream and the currant jelly. Be careful stirring the pot as to not break up the meatballs.  Bring the contents of the Dutch oven to a boil over medium-high heat, then reduce the heat so that the sauce simmers.  Cook, uncovered, for 45 minutes to 1 hour, or until the meatballs are very tender and the sauce is thick and flavorful.  

Meatballs are traditionally served with boiled new potatoes, cucumber salad and preserved lingonberries, but make a wonderful addition to any party spread.