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1
Place each pepper directly over a gas flame or directly under a hot grill. Turn as needed until the skin is completely charred on all sides. Transfer to a bowl, cover, and leave to steam for 10 minutes. Peel away the blackened skin entirely — no charred skin should remain — then remove the seeds and tear the flesh into broad strips. The flesh should be supple and bright red throughout.
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2
Warm the olive oil in a wide, heavy pan over low heat. Add the sliced white onion and a pinch of salt. Cook uncovered over low heat, stirring occasionally, for 15 minutes. The onion must be fully softened and translucent, with no colour taken on.
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3
Add the pepper strips, the crushed peeled tomatoes, the white wine vinegar, and the measured salt. Stir to combine. This is where the dish lives or dies: the heat must remain very low and the pan uncovered for the full duration — too high a flame burns the peppers and drives off liquid before the flesh can collapse; too short a time leaves the preparation loose and raw-tasting rather than dense and jammy. Cook for 45 to 60 minutes, stirring occasionally, until the peppers are almost falling apart and the sauce has reduced to a dense, deeply coloured stew. No standing liquid should remain.
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4
Taste and adjust salt. Remove from heat. The finished peperonata should hold its shape on a spoon yet spread slowly — the consistency is that of a thick jam. It may be served warm or at room temperature, and its flavour will deepen further after one to two days.