1. In a large mixing bowl, moisten the bread with the milk and using your hands, repeatedly squeeze and mash the bread until it's almost a paste. Drain off any excess milk and add the pork, veal, beef, mortadella, cheese, nutmeg, and 2 teaspoons of salt.
2. Beat two of the eggs and add to the bowl. Using your hands, work the mixture until all the ingredients are very thoroughly combined.
3. Pass the meat mixture back and forth between your hands to form one big, smooth, round meatball without any seams or air pockets. Next, shape the meat into a smooth loaf about 8 inches long by 5 inches wide and 3 inches high. Transfer to a plate, cover, and refrigerate for at least 1 hour.
4. In a skillet, warm 3 tablespoons of the olive oil over medium heat and cook the onion, carrot, celery, garlic, parsley, and sage, stirring occasionally, until the onion is pale gold at the edges, 8 to 10 minutes. Remove from the heat and set aside.
5. Preheat the oven to 325°F.
6. In a flameproof casserole large enough to hold the loaf easily, heat the remaining 4 tablespoons of olive oil over medium-high heat. Salt the chilled meat loaf on top and, when the oil is quite hot, carefully put it in the casserole, salted side down. Cook, undisturbed, until deeply browned, reducing the heat if it threatens to burn, 5 to 7 minutes (see Notes).
7. Salt the side of the meat loaf now facing up and, using a thin metal spatula, very carefully release the meat from the bottom of the casserole. With the help of a second spatula, turn the loaf over. Add the wine and bring to a simmer. Adjust the heat so the wine simmers gently and cook until reduced by one-third, 3 to 4 minutes. Stir in the water. Add the vegetable mixture, spreading some over the top of the meat loaf itself, and baste the meat generously.
8. Wrap a 15-inch-square piece of parchment paper or aluminum foil over the top and sides of the meat so that the paper or foil presses gently against its exposed surfaces. Let the excess extend over the vegetables. Cover the casserole and bake for 40 to 45 minutes, or until an instant-read thermometer inserted in the center registers 155°F.
9. Let the meat loaf rest for 15 minutes, covered in the casserole. Transfer to a cutting board and keep it covered with the parchment or foil.
10. Meanwhile, rewarm the liquid in the casserole over very low heat. Beat the remaining egg in a small bowl and whisk a ladleful of the warm cooking liquid into the egg to temper it without curdling it. Slowly pour the egg mixture into the casserole, whisking continuously for 1 to 2 minutes until the sauce is creamy and fluid. Do not let it boil. (If the sauce seems overly thick, thin it with a bit of water.) Remove from the heat and keep warm.
11. Spoon about one-third of the sauce on serving plates or a serving platter. Slice the meatloaf into 3/8-inch-thick slices and arrange the slices on the plate or platter, overlapping them. Spoon the remaining sauce over the slices and generously top with freshly ground black pepper. Serve immediately.