How to Make It
Lightly oil a cookie sheet. In a large bowl, mash the bread with the milk until a paste forms. Add the ground pork, veal, beef, then the mortadella, Parmesan and nutmeg. Season with 1 tablespoon of kosher salt.
In a small bowl, lightly beat 3 of the eggs and pour over the meat. Using your hands, gently work in the beaten eggs. Pat the meat into a large ball. Lift the meat from the bowl and gently toss it back and forth to compact it slightly. Transfer the meat to the cookie sheet and pat it into a 10-by-5-inch loaf. Cover with plastic wrap and refrigerate for 1 hour.
Meanwhile, in a large skillet, heat 1/4 cup of the olive oil until shimmering. Add the onion, carrot, celery, garlic, parsley and sage and cook over moderate heat, stirring, until softened and just beginning to brown, about 8 minutes.
Preheat the oven to 325°. Heat the remaining 1/4 cup of olive oil in a medium roasting pan set over a burner. Season the meat loaf with salt and carefully slide it into the roasting pan. Cook the meat loaf over moderately high heat, undisturbed, until browned on the bottom, about 7 minutes. Remove the pan from the heat and using a spatula, loosen the meat loaf from the roasting pan. Using 2 spatulas, carefully transfer the meat loaf to the cookie sheet, browned side up, then slide it back into the roasting pan.
Return the roasting pan to the burner, add the wine and cook over moderate heat until slightly reduced, about 5 minutes. Add the cooked vegetables, spreading some over the meat; spoon some of the liquid on top. Place a sheet of parchment paper directly on the meat loaf, pressing it all around. Cover the roasting pan tightly with foil and bake the meat loaf for 35 minutes, or until an instant-read thermometer inserted in the center registers 155°. Uncover and let the meat loaf rest in the pan for 15 minutes.
Transfer the meat loaf to a cutting board and cover loosely with foil. Set the roasting pan over low heat and add the water. Using a spatula, scrape up any browned bits stuck to the bottom and side of the pan and bring to a simmer. In a small bowl, lightly whisk the remaining egg. Measure out 1 cup of the hot pan juices and whisk them into the egg. Gradually whisk the egg mixture into the roasting pan, and whisk until the gravy is slightly thickened and creamy, about 2 minutes. Pour the vegetable gravy into a warm gravy boat and cover. Cut the meat loaf into 1/2-inch slices and serve with the gravy.