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1
Pat the scored fish dry and season on both sides with fine sea salt. Heat 2 tablespoons of the olive oil in a wide, heavy pan over high heat. Lay the fish in the pan and sear without moving until the skin is golden and releases cleanly from the surface — approximately 3 minutes per side. Transfer the fish to a plate and set aside.
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2
Reduce the heat to medium. Add the remaining olive oil to the same pan. Add the onion and cook, stirring, until softened and translucent — approximately 5 minutes. Add the garlic and cook for a further minute until fragrant but not coloured.
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3
Pour in the white wine and let it reduce by half, scraping up any residue from the fish. Add the tomatoes and cook, stirring occasionally, until they break down into a cohesive sauce and the raw tomato smell has left the pan — approximately 10 minutes.
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4
This is where the dish lives or dies: add the olives, capers, pickled jalapeños, jalapeño pickling liquid, and Mexican oregano. Stir once to distribute. The balance of brine, acidity, and tomato must be assessed here — taste and adjust salt before the fish enters the pan. If the sauce is overwhelmingly sharp or salty, it will only intensify; if it is flat, the fish will not rescue it.
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5
Nestle the seared fish back into the sauce, spooning the sauce over the top. Cover the pan and braise over low heat until the flesh at the thickest point of the scored cut is just opaque and yields to gentle pressure — approximately 15 to 18 minutes. Do not overcook; the flesh should pull from the bone cleanly but remain moist.
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6
Transfer the whole fish to a warmed platter and spoon the sauce generously over and around it. Serve immediately alongside plain white rice.