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1
Soak the sultanas in white wine for 15 minutes. Drain and reserve. Butterfly the sardines and remove the backbone, leaving the fillets joined at the tail. Pat thoroughly dry with kitchen paper. Season lightly with fine sea salt.
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2
Pour abundant olive oil into a wide, heavy pan to a depth of at least 1 cm and bring to high heat. Fry the sardines in uncrowded batches until deeply golden and crisp on both sides. This is where the dish lives or dies: sardines that are insufficiently fried will collapse to mush during maceration; they must emerge with a firm, golden crust that will hold through 48 hours in the marinade. Drain on a rack or kitchen paper.
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3
In a separate wide pan, warm a generous measure of olive oil over low-medium heat. Add the finely sliced white onions and cook slowly, stirring occasionally, for 30 minutes. The onions must be completely soft and translucent, with no colour. Add the white wine vinegar, sugar, pine nuts, and the drained sultanas. If using the older spiced version, add the cinnamon and clove now. Stir and cook for a further 5 minutes until the liquid is fragrant and the mixture coheres as a warm, loose saor. The smell shifts from sharp raw vinegar to a mellow sweet-sour scent.
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4
In a terracotta crock or deep ceramic dish, build the layers: begin with a layer of fried sardines, cover with a generous layer of warm saor — onions, fruit, nuts, and liquid together. Repeat, alternating sardines and saor, finishing with saor on top. Press the layers firmly so the warm saor liquid submerges or nearly submerges the fish. The surface should glisten with a visible covering of the agrodolce brine.
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5
Allow to cool to room temperature, then cover tightly and refrigerate for a minimum of 24 hours; 48 hours gives optimal flavour integration. Before serving, bring the dish to room temperature — the congealed fat of a cold sardine in saor is a fault. The sardines should be supple, deeply amber, and saturated with the sweet-acid-onion marinade.