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1
Arrange the cleaned whole fish in a single layer in a wide, heavy pan. Scatter the crushed garlic cloves evenly among the fish. Pour the olive oil and all 200 ml of white wine vinegar directly over the fish. Add no other liquid — no water, no wine, no broth. The vinegar and oil together form the sole cooking medium.
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2
Set the pan over a medium-high flame, uncovered. This is where the dish lives or dies: the pan must remain uncovered throughout so the vinegar reduces completely and forms a concentrated glaze around the fish — cover it and the steam prevents reduction, cover it and the dish fails.
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3
Add the coarsely ground black pepper in quantity once the liquid has begun to bubble. Continue cooking, uncovered, without stirring — allow the reduction to proceed undisturbed. The sound shifts from a sharp, rolling boil to a quieter, more viscous sizzle as the vinegar evaporates.
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4
The dish is done when almost all liquid has evaporated and the fish is coated in a dark, concentrated vinegar-oil glaze. The pan should appear nearly dry. Touch the glaze with a spoon: it should be dense and clinging, not pooled or watery.
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5
Transfer the fish to warmed plates. The dark residue remaining in the pan is reserved for mopping with bread. Serve immediately alongside soft polenta. Taste the glaze: it should be aggressively acidic, concentrated and pungent — any residual wateriness indicates insufficient reduction.