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1
Combine the beef, pork, chicken, and prosciutto in a wide pan with the carrot, celery, and onion. Cook over medium heat, stirring, until the meat colours. Add the white wine and braise briefly until the liquid has reduced and the pan is nearly dry. Pass the mixture through a fine grinder or chop to a uniform, fine texture. Spread on a tray and cool completely — the filling must be cold before binding.
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2
Transfer the cooled filling to a bowl. Add one egg, the Parmigiano, the nutmeg, and the lemon zest. Mix until the filling coheres into a firm, workable paste. It should hold its shape when pressed — neither wet nor crumbling.
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3
Pit each olive using a paring knife: begin at the stem end and cut in a continuous spiral around the stone, rotating the olive so the flesh comes away in a single unbroken strip. The olive is destroyed if it splits — take time on the first few. Remove the stone; the flesh strip remains intact.
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4
Form a small portion of the filling — roughly the volume of the removed stone — into a compact oval. Set it at the centre of the olive strip and wrap the flesh back around it in the same spiral, re-forming the olive's original shape. Press firmly so it holds. This is where the dish lives or dies: if the spiral cut was clean and the filling is cold and firm, the olive closes neatly; if either condition is not met, the olive will open in the fryer and the filling will be lost.
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5
Set up three shallow dishes: flour in the first, the remaining egg beaten with a splash of water in the second, fine breadcrumbs in the third. Roll each stuffed olive in flour, coat thoroughly in egg wash, then roll in breadcrumbs. Return each olive to the egg wash and breadcrumbs a second time. The double coat must be even and unbroken, with no bare patches.
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6
Heat the olive oil in a deep, heavy pan to 180 °C. Fry the olives in small batches — do not crowd the pan — for 2 to 3 minutes, turning once, until the crust is an even, deep golden colour. Lift out with a slotted spoon, drain briefly on kitchen paper, and serve immediately: the crust softens within minutes.