To make the filling, heat the oil in a large non-stick frying pan and fry the onion, carrots and celery for 6-8 minutes until the onion is softened and beginning to brown. Add the garlic and sausagemet and cook for 3-4 minutes, stirring constantly with a wooden spoon to break up any large clumps of meat.
Stir in the chilli flakes, oregano, sugar and plain flour and cook for one minute. Pour the red wine into the pan, followed by the tomatoes, stock and bay leaf. Bring to a gentle simmer and cook for 30-40 minutes, stirring occasionally until the meat is tender and the sauce has thickened.
For the cheese sauce, melt the butter in a large heavy-based saucepan and stir in the flour. Cook over a low heat for about 30 seconds, stirring constantly. Gradually stir in the milk. Increase the heat a little and bring to a gentle simmer. Cook for five minutes, stirring constantly.
Sprinkle over the parmesan and simmer for 1-2 minutes more until the cheese has melted. Season the sauce with salt and freshly ground black pepper. Remove from the heat and carefully cover the surface of the sauce with clingfilm to prevent a skin forming.
Preheat the oven to 190C/375F/Gas 5.
Wash the marrow well and put on a board. Cut in half horizontally and scrape out the seeds with a dessert spoon. Cut a thin sliver from the bottom of one of the halves so it sits flat on the board without rocking.
Place a sheet of kitchen foil in a large roasting tin and place the flat-bottomed marrow on top, with the deseeded side facing upwards. Spoon the meat mixture into the hollow and spoon the cheese sauce on top. Tear the mozzarella into pieces and arrange on top. Top with the remaining marrow to fully enclose the filling.
Bring the foil up around the marrow and pinch the edges to seal. Bake in the centre of the oven for 1-1¼ hours or until the marrow is tender and the filling is hot.
Carefully lift the marrow onto a serving platter or board and gently remove the foil. Serve the marrow in thick slices with a fresh green salad.