In a medium skillet over medium-high heat, saute 1 cup of the onions in 1 1/2 tablespoons of the butter until soft and light golden brown, about 4 to 6 minutes. Set aside to cool. Add the onions 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 nutmeg.
In a small bowl combine the breadcrumbs with the milk and 1/4 cup of the cream and allow to sit until bread crumbs are soft, about 5 minutes. Add the bread crumb 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 small portions, each about 1 heaping tablespoon. 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 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. 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 1/2 tablespoon of butter to the skillet before each addition.
Add the remaining 1 1/2 tablespoons of butter and the remaining 1/2 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 1/4 teaspoon salt, a pinch of black pepper, 3/4 cup heavy cream and the currant jelly. Be careful when stirring, 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.