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1
Beat the dried stockfish firmly on all sides with a wooden mallet. This breaks the fibres and allows even rehydration. Submerge in a large vessel of cold water and soak for 48 hours, changing the water every 12 hours. At the end of soaking, the fish should be fully pliable and significantly enlarged; it yields without resistance when pressed.
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2
Drain the stockfish and cut into large, even portions. Lay half the sliced onion across the base of a terracotta pot. Arrange the anchovy fillets, rosemary sprigs, olives, capers and lemon zest over the onion. Place the stockfish pieces in a single layer on top, then cover with the remaining onion, tomatoes and any remaining aromatics. Pour the white wine and olive oil evenly over the layers. Do not add water. The layered structure must remain intact throughout cooking; do not stir at any point.
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3
This is where the dish lives or dies: set the pot over the lowest possible heat — the surface must never boil, only tremble with the faintest, intermittent shudder. If the liquid approaches a boil at any point, the fish will break to shreds and the silky, gelatinous texture that defines this preparation is lost. Maintain this barely-trembling simmer for 2 to 3 hours. When the stockfish holds its shape but yields cleanly to a fork with no resistance, and the braising oil has taken on a deep amber, aromatic colour, the dish is ready.
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4
Remove the pot from the heat. To prevent sticking, shake the pot gently by its handles — never use a spoon or spatula inside. Allow the dish to rest, uncovered, for 10 minutes before serving. The braising liquid should have reduced to a rich, glossy emulsion that coats the fish.