Preheat the oven to 200C/400C/Gas 6.
Lay the pork fillet on a board and, with a sharp knife, cut through from the thick end to the pointed end, being careful only to cut halfway through the meat.
Cut into each side of the long cut, and fold back the flaps.
Place the mozzarella in the centre of the fillet all the way along. Spoon over a third of the chutney and scatter over the sage leaves. Season well.
Roll up the fillet and tie it with string. Rub the surface with the olive oil, salt and pepper.
Sear the outside of the pork, seam side down first, in a hot frying pan.
Place the fillet on a baking tray and surround it with the baby tomatoes. Bake in the oven for fifteen minutes until cooked. The fillet should be firm to the touch. Don't be tempted to cut it open to check as the filling will leak out.
Meanwhile, heat the butter in a small pan, add the spinach, and cook down until just wilted. Grate over the nutmeg.
Serve the pork with the remaining plum chutney warmed gently, the wilted spinach and the roasted baby tomatoes.