The panino con cipolla rossa di Tropea is the rare sandwich where a raw onion is the whole idea rather than a sharp accent buried under something else. The cipolla rossa di Tropea, the long violet onion grown on the Calabrian coast, is so low in the compounds that make ordinary onion harsh that it can be eaten in slices like a fruit, sweet and mild with only a faint bite at the back. The sandwich leads with that sweetness. Sliced thin and raw, or cooked down slowly into a dark jammy marmellata, the onion is the filling, and the bread is built around it rather than the other way round.
The craft splits along that fork: raw or jammed, and the two are different sandwiches with the same vegetable. Raw, the onion is sliced into thin rings, salted briefly and sometimes rinsed so it stays crisp and the sweetness comes forward, then dressed with oil and laid on bread with little more than a soft cheese or a slice of mild salume to ground it. Cooked, it is stewed low and long with a little sugar and vinegar until it reduces to a sticky sweet-sour marmellata that needs a sturdy bread and pairs against a sharp pecorino or a salted cured meat that the sweetness can push against. Either way the onion is not seasoning the sandwich, it is the subject, and the supporting parts are kept few and salty so the sweetness has something to lean on.
The variations are about that raw-or-jammed choice and what is allowed to join it, each its own preparation rather than a footnote. There is the build with fresh Tropea against 'nduja, the soft hot Calabrian paste, where sweetness meets fire, and the cheese-and-honey version that pushes the sweet register further still. The wider family of Calabrian vegetable panini follows the same logic of a developed regional ingredient leading the sandwich, and each deserves its own article rather than being crowded in here.