Adding onion to the sausage fold introduces the one thing a plain salsiccia piadina lacks, which is sweetness, and that changes the whole balance. Pork sausage on its own is savoury and fat from edge to edge; sauteed onion, cooked slow until it slumps and caramelises, threads a soft, sugary, slightly tangy line through it. The warm Romagnolo round ties the two together: its blistered, pliable crumb soaks the rendered sausage fat and the onion's syrupy juices into one base rather than letting them sit in separate puddles. The defining fact is that the bread is not a neutral carrier here but the surface where a fatty meat and a sweet vegetable are made to read as a single thing.
Making it well depends on the onion being properly cooked and the fold staying intact. The dough is the usual Romagnolo mix, flour with lard or oil and barely any yeast, rolled thin and cooked dry on a hot plate until it freckles and stays foldable, lifted before it crisps so it bends around a wet, bulky filling without splitting. The onion wants long, gentle cooking, soft and golden and almost jammy, not rushed and raw and sharp, because raw onion would only fight the sausage instead of rounding it. The salsiccia is split and browned on a wide face, then the two are layered warm so the heat marries them. A little soft fresh cheese is sometimes added to bind and calm the whole. It is folded in half and eaten hot, before the fat and the onion juice cool and slick the crumb.
The close cousins are one element from here and stay regional. There is the plain version with the sausage alone, the one that swaps sweet onion for bitter greens cooked down beside the meat, and the fold built on a spicier southern sausage with the onion left in. Each is the same warm round meeting a single changed element, and each deserves its own article rather than being crowded in here.