This Neapolitan panino is defined by its bitterness, not its sausage. Friarielli are the leafy tops and buds of a local broccoli relative, wilted hard in oil with garlic and chilli until they collapse into a dark, faintly bitter, slightly mineral tangle. Paired with a coarse pork salsiccia, the greens are not a side dressing but the equal half of the sandwich: the sausage brings fat and salt, the friarielli bring a green bitterness that cuts straight through it. Take the greens away and you have a sausage roll like any other. The whole point of the pairing is that each one makes the other bearable in quantity.
The craft is in cooking both elements until they meet in the middle. The sausage is grilled or pan-cooked so its fat renders and its casing colours, then often split lengthwise so the cut faces caramelise and the meat lies flat against the bread. The friarielli are cooked down properly rather than merely warmed, because underdone they read harsh and stalky; the garlic and chilli are carried in their oil, and that oil is what dresses the bread. The two are layered hot so the sausage fat and the bitter greens bleed into each other and into the crumb. The bread is a plain roll or a length of soft pizza-style dough, sturdy enough to take the oil without going to paste. It is eaten warm, when the fat is still loose and the bitterness still sharp.
The variations stay in Campania and mostly swap the carrier or add a melt. There is the version built into a panuozzo and returned to the oven, the one with provola laid over the sausage so the cheese strings into the greens, and the friarielli served alone on bread for the meatless table. The panuozzo and the provola build each pull the sandwich somewhere distinct enough that they deserve their own articles rather than being crowded in here.