The Panino Bolognese is a sandwich that reads as Bologna before anything else, and the thing that makes it Bolognese is mortadella. This is the city's defining salume: finely ground pork emulsified to a pale, silky paste, studded with cubes of white back fat and often pistachio, cased wide and gently cooked until it is soft, round, and faintly sweet, with none of the salt-edge or chew of a dried meat. A generous fold of it on the local bread is the panino. Where Abruzzo's regional roll leads on a coarse, fiery ventricina and a dense country loaf built to resist it, Bologna leads on the opposite instinct entirely: a smooth, mild, almost delicate meat that asks the bread to be soft and to get out of its way.
The craft is in handling that softness rather than fighting it. Mortadella wants to be sliced thick and then folded loosely into the roll rather than laid flat, so that air lifts it and it reads as a tender, billowing thing instead of a packed slab; some stands warm it slightly so the back fat just begins to release and the sweetness opens. The bread is chosen to match: a soft rosetta or a tender crumb that compresses to the meat, never a hard crust that would overpower so gentle a salume. Almost nothing else belongs, because mortadella is already complete and a strong dressing would flatten it; at most a plain bread is kept plain and the quality is decided at the Bolognese salumeria counter where it was cut.
The variations stay within the Bolognese and wider Emilian larder, and each is its own preparation rather than a footnote here. The pistachio-heavy mortadella read on its own, the same meat paired with a soft local cheese, the regional salame of the Emilia hills on the same soft bread, the warmed reading where the fat is allowed to slacken. Each of those is a distinct balance on the city's own counter, and each deserves its own article rather than being crowded in here.