Place the bread crumbs into a small bowl, and mix in the cream. Allow to stand until crumbs absorb the cream, about 10 minutes. While the bread is soaking, melt 1 teaspoon of butter in a skillet over medium heat, and cook and stir the onion until it turns light brown, about 10 minutes. Place onion into a mixing bowl; mix with the ground beef, ground pork, egg, brown sugar, salt, black pepper, allspice, and ginger. Lightly mix in the bread crumbs and cream.
Melt 1 tablespoon of butter in a large skillet over medium heat. Pinch off about 1 1/2 tablespoon of the meat mixture per meatball, and form into balls. Place the meatballs into the skillet, and cook just until the outsides are brown, about 5 minutes, turning the meatballs often. Insides of the meatballs will still be pink. Place browned meatballs into a baking dish, pour in chicken broth, and cover with foil.
Bake in the preheated oven until the meatballs are tender, about 40 minutes. Remove meatballs to a serving dish.
To make brown gravy, pour pan drippings into a saucepan over medium heat. Whisk the flour into the pan drippings until smooth, and gradually whisk in enough beef broth to total about 2 1/2 cups of liquid. Bring the gravy to a simmer, whisking constantly until thick, about 5 minutes. Just before serving, whisk in the sour cream. Season to taste with salt and black pepper. Serve the gravy with the meatballs.