Preheat the oven to 220ºC/425ºF/gas 7. Cut away and discard the ciabatta crusts, tear into chunks and place in a bowl. Cover with cold water and leave to soak. Meanwhile, finely slice the mortadella, 2 garlic cloves and the parsley leaves, then place into a large bowl with the mince, eggs and grated Parmesan. Squeeze the water out of the bread and add to the bowl.
With wet hands, scrunch and mix everything together, season well with salt and pepper, then divide and roll into 12 equal-sized balls. Place in a roasting tray (about 25cm x 30cm), drizzle over a little olive oil and place in the fridge for about 10 minutes to firm up.
Meanwhile, finely chop the remaining garlic cloves, the onion and chilli. Drain and finely chop the peppers, reserving the brine. Place the meatballs into the hot oven for 15 to 20 minutes, or until golden, shaking the tray from time to time to stop them from sticking, then place directly over a low heat on the hob. Add the wine, allow it to boil and bubble away, then stir in the onion, garlic and chilli and cook for a few minutes, or until turning golden. Stir in the pepper and brine, and the plum tomatoes, breaking them up with the back of a spoon, then season and add a splash of balsamic vinegar.
Return the tray to the hot oven for 20 to 30 minutes, or until the meatballs are cooked through (if the meatballs start to golden too quickly, cover the tray loosely with tin foil). Cut the mozzarella into 12 slices, then carefully place on top of each meatball and return to the oven for a further minute, or until melted.
Open out the bread rolls, fill each with a cheesy meatball, spoon over the tomato sauce, then add a drizzle of balsamic vinegar. Push down on the sandwich to squidge it shut, then tuck in.