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1
Place the raisins in warm water and leave for 10–15 minutes until visibly swollen. Drain and set aside. Dry raisins added directly to the sauce will absorb its liquid and distort the agrodolce balance.
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2
Warm 50 ml of the olive oil in a wide, heavy pan over a medium flame. Add the onion and celery and cook, stirring occasionally, until soft and golden but not brown — a pale, yielding mass that yields no bitterness. This takes approximately 10 minutes.
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3
Add the passata, capers, olives, drained raisins, vinegar, sugar, and orange peel strip. Stir to combine. Simmer uncovered over a low flame for 10 minutes until the sauce thickens to a coating consistency. Taste and adjust: the balance must be complete before the fish enters — this is where the dish lives or dies. Add vinegar for sharpness or sugar for sweetness, a little at a time, until the sweet-sour equilibrium is resolved.
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4
Pat the swordfish steaks dry. Dredge lightly in flour, shaking off any excess. Heat the remaining 30 ml of olive oil in a separate frying pan over a high flame until shimmering. Sear the steaks for 90 seconds per side — the surface should be lightly golden and the flour just set. The fish is not cooked through at this point; the sauce completes the cooking.
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5
Remove the orange peel from the ghiotta sauce. Lay the seared steaks into the sauce in a single layer. Cover and braise over a low flame for 8–10 minutes. The fish is done when it yields to gentle pressure at the thickest point without resistance, and the flesh has just lost its translucency at the centre.