Tear bread into small pieces and transfer to bowl of food processor. Process until finely ground; set aside. Bring a 4-qt. saucepan of salted water to a boil. Add spinach and cook, stirring, until just wilted, 15–30 seconds. Using a slotted spoon, transfer spinach to a bowl of ice water. Drain spinach and squeeze it with your hands to remove excess water. Set spinach aside.
Heat oven to 400°. In a large bowl, combine the bread crumbs with the beef, pork, mortadella, cheese, rosemary, nutmeg, beaten eggs, garlic, and onions, and season with salt and pepper. Transfer half of the meat mixture to a 9" x 13" baking dish. Using your hands, form meat mixture into a 4" x 8" rectangle about 1" thick. Arrange half of the spinach on top of rectangle in a 2"-wide strip down the middle. Arrange the hard-boiled eggs end to end on top of the strip of spinach, and top eggs with remaining spinach. Using your hands, shape the remaining meat mixture on a sheet of parchment paper into a rectangle roughly 4" x 8" and lay it over the spinach, eggs, and meat. Press the meat gently around the eggs to form a uniform loaf, pinching top half of meat together with bottom half and smoothing any seams.
Brush meat loaf with oil and bake for 30 minutes. Pour in broth and wine and continue cooking meat loaf, basting every 10 minutes or so with liquid, until an instant-read thermometer inserted into the loaf registers 155°, about 20 minutes more. Using a spatula, transfer meat loaf to a cutting board and let rest for 5 minutes. Strain cooking liquid; set aside. To serve, cut meat loaf into thick slices, transfer to a serving platter, and spoon some of the reserved cooking liquid over the top.