The rosetta con mortadella turns on two things that need each other: a hollow roll and a soft pink sausage that fills its emptiness without weighing it down. The rosetta is the Roman ring roll, shaped into five raised petals around a domed centre, baked with a thin, brittle, papery crust and almost no crumb, so a split rosetta opens onto an air chamber rather than a dense interior. Mortadella is the wide Bolognese cooked pork sausage, finely emulsified, studded with white fat cubes and often pistachio, sliced into floppy sheets that drape rather than stack. The defining fact is the fit between the two: a bread that is mostly shell and space, and a filling so supple and light that it can line that space without crushing the walls. Without the mortadella the rosetta is an empty crackle; without the hollow the mortadella has nowhere soft to fold into. The pairing reads as shell, then air, then a cushion of cool sausage.
The craft is in respecting how little the shell can carry and how fast it goes. A rosetta shatters under the teeth, which is its appeal, but that crust loses its snap the moment a moist filling sits against it for long, so the roll is filled close to eating rather than built ahead and held. The mortadella is sliced thin and ribboned loosely into the chamber, folded into soft waves so it traps a little air and the bite stays light, never packed flat into a dense slab that would press the petals apart and turn the roll soggy at the seam. Nothing wet is added, because the whole point is the dry contrast of brittle crust against yielding sausage; the salt and the gentle fat come entirely from the meat. A sloppy version overstuffs a tight or stale roll and collapses its own walls; a good one keeps the chamber partly hollow and lets the crust do the talking. This is forno food, bought in the morning and eaten before the shell gives in.
The close cousins are mostly arguments about what goes into the chamber, and they belong to the fillings rather than to the roll. There is the rosetta con prosciutto built on cured ham, the prosciutto cotto and plain cheese builds, and the regional relative of the roll itself, the Milanese michetta, the same hollow-shell idea baked under a different sky. Each is a distinct filling or a distinct bread with its own logic, and each deserves its own article rather than being crowded in here.