Take it from Batali, no ordinary food has to be entirely ordinary. Especially when it involves stuffing something with something else.
- Preheat the oven to 400°F. In a large bowl, combine the sausage, beef, 4 cups of the breadcrumbs, the pecorino, eggs, milk, and salt and pepper.
- Mix gently but thorougly with your hands. Cover and refrigerate.
- Bring 8 quarts of water to a boil in a large pasta pot. Set up an ice bath nearby.
- Add 2 tablespoons salt to the boiling water. Dip the spinach leaves in the water just to wilt them, immediately transfer them to the ice bath, then drain them in the colander.
- Add the carrots to the boiling water and cook for 10 minutes, then remove with a spider or slotted spoon and set aside. Drop the scallions into the boiling water and cook for 1 minute.
- Drain and set on a towel-lined plate.
- Combine the flour with the remaining 1/4 cup breadcrumbs, and heavily dust a wooden board or work surface with the mixture.
- Place the spinach leaves between two plates and press them together to remove remaining water, and then lay the spinach over the meat, leaving a 1-inch border on the short sides.
- Lay the carrot pieces and then the scallions over the spinach, arranging them lengthwise down the rectangle.
- Lay the mortadella and Cacio di Roma over the scallions.
- Starting from a long side, roll the meat up like a jelly roll, making it as compact as possible; patch any holes like modeling clay. The roll should be about 16 inches long. Dust the outside with flour.
- Place the loaf (on a diagonal if necessary) on a rimmed baking sheet, and press a sprig of rosemary onto each side of the polpettona and pour 1 cup of water into the pan.
- Drizzle 1/2 cup of the olive oil down the length of the loaf. Bake the polpettona for 1 hour, or until it reaches an internal temperature of 165 F.
- Carefully transfer the loaf to a cutting board and allow it to rest for 15 minutes.
- Strain pan juices into a saucepan and bring to a boil, season with salt and pepper to taste, add the remaining 1/4 cup oil and whisk to form a loose sauce.
- Slice the meatloaf into 1 inch-thick slices, drizzle with sauce and serve.