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1
Combine the water, white wine, carrot, celery, onion, bay leaf, peppercorns and salt in a pot large enough to hold the veal. Bring to a simmer, then lower the veal into the liquid. Regulate the heat so the liquid holds at 70–75 °C throughout — it must never boil. Poach until a probe thermometer inserted at the centre reads 65 °C. The meat is done when it offers no resistance to the probe and remains pale throughout; any greyness indicates overcooking.
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2
Remove the pot from the heat. Leave the veal to cool completely in its poaching liquid — this is where the dish lives or dies: meat lifted from the liquid too soon will dry out and lose the aromatic depth that only a full cooling in the bouillon can give. Once at room temperature, refrigerate the veal in its liquid until thoroughly cold, at least four hours.
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3
Lift the cold veal from the liquid. Pat dry. Using a sharp slicing knife, cut across the grain into slices 2–3 mm thick. The slices should be translucent at their edges and hold together without tearing.
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4
Place the drained tuna, anchovy fillets and rinsed capers in the bowl of a food processor. Process until completely smooth. Add the mayonnaise and pulse to combine. The sauce must coat a spoon thickly and hold its shape on the veal without running; if it is too loose, incorporate additional mayonnaise a tablespoon at a time until the correct consistency is reached.
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5
Arrange the veal slices in overlapping layers on a large flat serving dish or airtight container. Spread the tonnata sauce generously over every layer and over the surface, ensuring no veal is left exposed. Cover tightly and refrigerate overnight. The sauce must have time to penetrate the veal; the flavours are not fully integrated until the following day.
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6
To serve, transfer to a clean platter if necessary. Scatter the whole small capers over the surface and finish with a drizzle of extra-virgin olive oil. The dish is ready when the sauce has taken on the faintest blush of the veal beneath it and the two elements smell as one.