Prepare the broth by boiling the vegetables in the water, over a moderate heat, for roughly 20 minutes, then filtering to remove the pieces.
Using a very sharp knife that’s slightly longer than the fillet, create a hole, lengthwise, right through the centre of the meat (rotate the knife inside the fillet to widen the hole). Insert 15 prunes into the hole, one after the other, pushing them in first with your fingers and then with the handle of a wooden spoon, so they go all the way in. Season the fillet all over with salt and pepper, tie it up with kitchen string and insert a sprig of rosemary and a sprig of sage in the string both on top of and below of the meat.
In a deep, heavy-bottomed pan, sauté the cloves of garlic, squashed with the palm of your hand, in the olive oil; when they are golden brown, put the meat in as well.
Brown all over for about 10 minutes over a lively heat, remove the cloves of garlic, turn up the heat and pour in the wine. When it has evaporated completely, add the remaining prunes and the broth. When it comes to the boil, turn the heat down, put the lid on the pan – leaving a small gap – and simmer slowly for an hour, turning the fillet over from time to time. When the meat is cooked, lay it on a chopping board and let it cool. In the meanwhile, filter the cooking juices (including the prunes) through a sieve, squashing with the back of a spoon; then add the grated peel of half an orange and a little orange juice to the sauce. Remove the string from the pork fillet and cut it into slices about 1 cm thick, revealing the surprise in the centre – the prunes! Arrange the slices on a serving dish and serve accompanied by the sauce.