Pizza bianca con mortadella is the Roman bakery reflex of taking a sheet of warm oiled bread straight off the oven floor, splitting it through the side, and laying in cool slices of mortadella. What defines it is the temperature contrast and the way the two parts trade fat. The pizza bianca is a flat, blistered, salty, olive-oiled bread with a chewy interior and a crackling skin; the mortadella is a soft, pale, gently spiced pork sausage shot with cubes of white fat. Slide the cold meat into the hot bread and the residual warmth slackens that fat just enough to release its scent and let it seep into the crumb. Without the warmth the sandwich is correct but inert; without the mortadella it is only good bread. The pairing is built so each loosens the other.
Making it well starts at the bread. The pizza bianca wants to be used soon after it comes out, still warm and pliable, brushed with oil and scattered with coarse salt so the surface tastes seasoned on its own. It is split horizontally with a wide pocket, not torn, so the slices lie flat rather than bunching. The mortadella is sliced thin and folded loosely into waves rather than stacked in a flat slab, which keeps the filling airy and lets the bread close without crushing it. Quantity is judged against the salt already in the pizza bianca: enough meat to read clearly, not so much that the combined salt overwhelms. A sloppy version uses cold hard bread, a mean sliver of meat, and a flat tight stack that turns greasy where the fat sits in one place; a good one is warm, generous, and loosely layered so the fat is spread thin across the whole bite.
Close relations stay within the same Roman bakery logic and each is its own subject rather than a footnote here. There is the pizza bianca con prosciutto, where a salt-cured raw ham replaces the soft sausage and changes the whole register, the version finished with a few fresh figs pressed against the meat in season, the mortadella studded with pistachio swapped in for the plain, and the build on a sturdier rosetta roll instead of the flatbread. Each is the same warm-bread-and-cured-pork idea with one element changed, and each deserves its own article rather than being crowded in here.