The panino con bigoli in salsa is a Venetian carb-on-carb proposition: a cooked pasta dish put inside bread, with all the friction that implies. Bigoli are thick, rough, extruded wholewheat noodles, fatter than spaghetti and porous enough to grip a sauce, and in salsa means dressed with the Venetian standard of onions cooked down slow and soft with dissolved anchovy and oil until the whole thing is a savoury, almost jammy coat. As a plate it is one of the great simple dishes of the lagoon. Forced into a panino it becomes a study in starch against starch, soft noodle inside soft crumb, held together by the salt and onion of the sauce. That is the sandwich, and it does not pretend the combination is conventional.
The craft is making a slippery, sauced pasta behave between bread. The bigoli are kept short of fully soft and dressed thoroughly so the onion-anchovy sauce clings to every strand rather than pooling at the bottom of the roll; an under-sauced filling slides apart and an over-sauced one floods the crumb. The bread is chosen with a real crust and a firm interior so it gives the bite some resistance, because two soft elements with nothing structural between them read as mush, and the crust is the only contrast the sandwich has. The sauce is the working part: the onions carry sweetness, the anchovy carries the salt and depth, and together they season the noodles strongly enough that the whole thing tastes finished without anything added. It is eaten soon, while the bread still holds and the pasta has not gone cold and claggy.
The variations stay Venetian and each is its own preparation rather than a footnote here. There is the version with the sauce reduced harder for a drier, more manageable filling, the one with extra anchovy for a sharper salt, the one built on a denser roll to fight the softness. Other Venetian preserved-fish and pasta traditions, the baccalà mantecato and the sarde in saor among them, follow their own logic and deserve their own articles rather than being crowded in here.