2. For the filling: Heat oil in a medium skillet over medium heat. Add onions and garlic and cook, stirring often with a wooden spoon, until onions are soft, about 5 minutes. Add chicken livers (if using) and mash to a paste with the back of the spoon. Add chicken, veal, pork, and salt and pepper to taste. Cook, stirring often, until meat is cooked through, about 10 minutes. Transfer meat to a cutting board and chop until texture is very fine and meat begins to hold together. Transfer meat to a medium bowl, stir in nutmeg, and set aside.