Fry onion, carrots, and celery in oil for 6-8 minutes until softened.
Add garlic and sausagemeat, cooking for 4 minutes while breaking up clumps.
Stir in chilli, oregano, sugar, and flour, and cook for one minute.
Add wine, tomatoes, stock, and bay leaf, then simmer for 30-40 minutes.
Melt butter in a saucepan, stir in flour, and cook for 30 seconds.
Gradually whisk in milk and simmer for 5 minutes until thickened.
Stir in parmesan, season with salt and pepper, and cover with clingfilm.
Preheat the oven to 190°C.
Halve the marrow horizontally and scrape out the seeds.
Place marrow in a foil-lined tin and fill the cavity with meat mixture.
Top with cheese sauce and torn mozzarella, then replace the marrow lid.
Seal the foil and bake for 60-75 minutes until tender.