Venice — sarde in saor is documented in Venetian sources from the 14th century as a preservation technique for fried fish. The Arabic sweet-sour-spice preparation arrived in Venice through the spice trade and was naturalised into the Venetian cucina as saor. It is now the emblematic cicchetto of the Venetian bacaro (bar).
Sarde in saor is the definitive Venetian cicchetto (bar snack) and one of the great preparations of the Italian agrodolce tradition — fresh sardines fried in olive oil, then marinated in a sauce of slow-cooked onions (white and sweet), white wine vinegar, pine nuts, raisins, and sometimes a piece of cinnamon. The preparation requires 24-48 hours of marinating before eating — the sardines absorb the sweet-sour-aromatic sauce during this period and become something quite different from freshly fried fish. The preparation is ancient (14th century Venice documents it as a way of preserving fried fish for sailors) and reflects the influence of the Arab-Byzantine spice trade through the Venetian Republic.
Sarde in saor after 48 hours of marinating are one of the most complex cold fish preparations in Italian cooking — the sardine flesh has absorbed the sweet-sour onion sauce; the raisins have plumped in the vinegar; the pine nuts add crunch; the cinnamon (if present) adds a warm, slightly exotic note from Venice's spice trade past. At room temperature on crostini, it is the Venetian bacaro at its best.
Flour sardines (cleaned, butterflied, or whole — traditions vary); fry in abundant olive oil at 175°C until golden on both sides. Drain; arrange in layers in a glass or ceramic container. For the saor: cook very thinly sliced white onions in olive oil over very low heat for 30-40 minutes until completely soft and sweet (not coloured). Add white wine vinegar; reduce by half. Add raisins (soaked in warm water), pine nuts (toasted), and optionally a piece of cinnamon. Pour the warm saor over the sardines; allow to cool. Refrigerate 24-48 hours minimum before serving. Serve at room temperature.
Some Venetian preparations use a small amount of sugar added to the saor for a more pronounced agrodolce; the traditional preparation relies on the onion's natural sweetness and the raisin's fruit. The saor sauce can be made with different fish (mackerel, lagoon fish, soft-shell crab in the cicchetti tradition) — the onion-vinegar-raisin-pine-nut saor is the constant. The preparation keeps refrigerated for up to a week.
Not waiting the full marinating time — the preparation eaten immediately after making is good but not saor; the 24-48 hour marinating is what produces the distinctive character where the sardines have absorbed the sweet-sour sauce. Frying the sardines in olive oil that is not hot enough — the sardines will be oily and soft rather than golden and crispy. Using malt vinegar — only white wine vinegar is correct.
Oretta Zanini de Vita, Encyclopedia of Pasta; Slow Food Editore, Veneto in Cucina; Giorgio Locatelli, Made in Italy