The sardines in this Venetian panino are not cooked to be eaten hot; they are cooked to be marinated and waited on. Sarde in saor are fried sardines layered with a sweet-sour onion mixture, slowly cooked onions slackened in vinegar with sugar, often pine nuts and raisins folded through, then left to sit a day or more so the fish drinks the marinade. The defining fact is that the dish improves by standing, which makes it a strange and excellent sandwich filling: the fish arrives already seasoned all the way through, soft, tangy, and stable, needing nothing added at the bread.
The craft is balance struck long before assembly. The sardines are fried first so they hold their shape under the marinade rather than disintegrating into it; the onions are cooked down properly until sweet, then the vinegar is added to pull them sharp, the saor finished so neither the sweet nor the sour wins. Time does the rest, the acid working into the fish until it is yielding and evenly flavoured. On bread the only problem is moisture: the fish and onions are lifted from their liquid and drained so the sandwich is dressed, not soaked, and the bread is a plain crusted roll firm enough to carry a wet, soft filling without going to paste. No further seasoning is added because the saor is the entire balance and anything more would unbalance it. It is eaten cool, never hot, when the marinade reads brightest.
The variations are mostly about what rides in the saor rather than the method. There is the plainer Venetian build with just onion and vinegar, the richer one with pine nuts and raisins worked through, and the related bigoli in salsa and other Veneto preserved-fish preparations. The Sicilian sarde a beccafico, stuffed and baked rather than fried and marinated, is a separate preparation entirely, and those deserve their own articles rather than being crowded in here.