In a small bowl, combine the bread and milk and let stand for 10 minutes. Drain and discard any milk that hasn't been absorbed by the bread. Tear the bread into ¼-inch pieces. Reserve.
In a food processor, combine the mortadella and prosciutto and pulse until the meat is coarsely ground--about 8 one-second pulses.
In a large mixing bowl, combine the mortadella and prosciutto with the ground pork, Parmesan, eggs and bread. Add the nutmeg and season the mixture well with salt and pepper. Mix gently by hand until just combined. Let the mixture rest in the refrigerator for 1 hour before forming the meat into about 14 golf-ball-size meatballs.
In a large, nonstick skillet, heat the olive oil over medium-high. Add the meatballs and cook, turning occasionally, until browned on all sides, about 10 minutes. Cover the meatballs with the marinara sauce, bring to a simmer over medium-low heat and cook, partially covered, until the meatballs are cooked through, about 20 minutes longer. Serve immediately.