A food processor would make things easier to chop the vegetables, prunes, and the mortadella.
Preheat the oven to 175°C/350°F. Lightly oil a baking dish or sheet.
Prepare the vegetables: Finely chop the onion, garlic, carrot, and celery. Heat the oil in a pan over medium heat, then add the vegetables and sauté 5 min, with occasional stirring. Add a little salt, cover, then lower the heat. Cook 5 min.
Take the pan off the heat, then add the Worcestershire sauce, vinegar, salt, pepper, and allspice. Finely chop the prunes, parsley, and mortadella (or ham). Add this mixture to the vegetables.
In a large bowl, beat the eggs lightly, using a fork, then add the beef, pork, bread crumbs, and milk. Mix well, then incorporate the vegetable mixture. Using your hands, blend the mixture together until it is just combined well (do not over-mix). In the prepared dish, form the mixture into a loaf, about 14-15 cm wide.
Cook in the middle of the oven 1 h to 1 h 15 min until a thermometer inserted into the centre of the meatloaf registers 68°C/155°F. Let the meatloaf stand 5-10 min before slicing and serving.