How to Make It
In a large bowl, combine the meats with the eggs, bread crumbs, currants, pine nuts, capers and three-fourths of the scallions. Add 1 tablespoon of salt and 1/2 teaspoon of pepper and knead gently to combine. Roll the mixture into 18 meatballs and dust with flour.
In a large skillet, heat the olive oil until shimmering. Add the meatballs in a single layer and cook over moderate heat, turning once or twice, until browned and nearly cooked through, 7 to 8 minutes. Tilt the skillet and spoon off as much fat as possible.
Add the remaining scallions to the skillet and cook for 1 minute. Add the broth, vinegar and sugar and season lightly with salt and pepper. Cover partially and cook over moderately low heat, stirring, until the sauce is reduced to 1/2 cup and the meatballs are fully cooked, 8 minutes. Serve with crusty bread.
The cooked meatballs and sauce can be refrigerated overnight.