Finely chop the onion, celery, carrot then the bell peppers and mushrooms.
Heat 1 Tbsp olive oil in a heavy-based dish then gently sauté the onion, celery and carrot. After about 5 minutes add the mushrooms and peppers.
Now add the garlic and the spices and mix well for a minute or so.
Add the mince, stir continuously until it changes colour then add the tomato purée and season with salt and pepper.
Let it cook for about 10 minutes until the sauce gets thick.
In the meantime set the oven to 160°C.
Cut the marrow in half (lengthwise) and, using a spoon, scrape out the fibrous flesh. Put the hollowed marrow halves in a baking tray and spoon the mince sauce into them. Scatter some cheese on top, if liked, and bake for 25-30 minutes until the marrow flesh is tender.
Serve warm!