The panino con genovese contains no basil, no pine nuts, and no pesto, and the name misleads almost everyone who has not eaten in Naples. La genovese is a Neapolitan sauce: a vast quantity of onions cooked down for hours with a tough cut of beef until the onions collapse into a sweet, brown, unctuous jam and the meat shreds into it. The dish is the point and the panino is its second life, the Sunday pot turned into something you can hold. The defining fact is that this is a stew in bread, not a spread, and what is doing the work is the slow onion, not anything sharp or green the name might suggest.
The craft is making a long-cooked wet thing behave between two pieces of bread. The genovese is reduced until the onions are almost a paste and most of the liquid is gone, because a soupy sauce would dissolve the crumb before the second bite; the beef is pulled into shreds so it distributes evenly rather than sitting as a block that drags out of the roll. The bread is chosen sturdy and is often a rosetta or a piece of pane casereccio, sometimes lightly toasted so the cut faces firm up against the moisture and the sweetness. The portion is controlled on purpose: enough to taste the depth of the onion, not so much that the sandwich fails at the seam. Nothing is added, because the sauce has already been seasoned over hours and a condiment would only blur it.
The variations are narrow and Neapolitan, each its own preparation rather than a line here: the classic beef genovese, lighter pork or veal versions, and the broader Campanian habit of spooning a finished braise into a roll, which is the Dish in a Panino and its own piece. Each is a slow sauce given a handle, and each deserves its own article rather than being crowded in here.