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1
Place the oxtail pieces in a large bowl, cover with cold water, and soak for 2 hours. Drain thoroughly and pat dry with a clean cloth. The water drawn off will be visibly pink; this purge is complete when the water runs clear.
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2
In a heavy-based casserole over high heat, sear the oxtail pieces in batches until deeply browned on all sides. Remove and set aside. Lower the heat, add the onion, whole garlic cloves, and the 5–6 roughly chopped celery stalks, and sweat until softened and lightly coloured — the celery must be present in abundance, not merely suggested.
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3
Return the oxtail to the casserole. Add the cloves and pour over the white wine. Raise the heat briefly to drive off the alcohol — the sharp edge of raw wine should leave the nose entirely — then add enough water to come three-quarters up the meat.
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4
Reduce the heat to the barest possible simmer — a single lazy bubble breaking the surface every few seconds. Braise uncovered for 4 to 5 hours, skimming the fat that rises to the surface at regular intervals, and turning the oxtail occasionally. This is where the dish lives or dies: any sustained higher heat converts the collagen to tough cartilage rather than the required gelatinous collapse, and no subsequent cooking will retrieve it. The meat is ready when it yields completely to the touch of a spoon and begins to part from the bone without resistance.
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5
With 30 minutes of braising time remaining, stir in the cocoa powder, drained sultanas, pine nuts, and the two fresh sliced celery stalks. These are not optional — they are what separates this dish from all other braised oxtail and marks it unmistakably as Roman. The sauce will deepen to a dark, bittersweet, glossy finish.
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6
Remove from the heat. If serving the following day — which is strongly preferable — allow to cool completely, refrigerate overnight, and lift off any solidified fat. Reheat gently. The sauce should set to a trembling, gelatinous consistency when cold and coat the oxtail in a dark, glossy film when warm. Serve with crusty bread for sauce-mopping; pasta is not the vehicle here.