Bring the stock to the boil and stir in the rice. Reduce the heat, cover and cook gently for 20–25 minutes, or until the rice is cooked and the stock is absorbed.
Meanwhile, preheat the oven to 200°C. Cut a slice about 3 cm thick from each end of the marrow and, with a dessertspoon, remove all the seeds to leave a hollow along its entire length. Set aside.
Heat the pine nuts gently in a dry, heavy-based frying pan until they are lightly browned, then remove from the pan and set aside. Melt half the butter with the oil in the frying pan, add the onion and cook until just soft. Stir in the celery and garlic and cook for 5 minutes. Remove from the heat and add the cooked rice, pine nuts, half the mint and the chopped tomatoes. Season well and mix together. Spoon this rice ﬁlling into the marrow, pressing it in well.
Spread the remaining butter over a piece of foil large enough to wrap around the marrow. Sprinkle it evenly with the remaining mint, then season it well. Place the marrow in the centre of the foil, wrap it up tightly and place it in a roasting pan. Bake in the oven for 1–11/4 hours, or until the marrow is tender.
Remove from the oven, unwrap and leave to stand for 5 minutes. Cut into 12 even slices and arrange on a serving dish or on individual plates. Garnish with fresh mint leaves and serve with homemade tomato sauce, if using.