For the filling
Preheat the oven to 180°C/350°F/Gas mark 4.
Peel the marrow and cut into 8 slices. Discard the pith and seeds. Put in a single layer in a baking tin, and sprinkle with salt and pepper. Add 2 tablespoons water.
Cover with foil and bake in the oven for 30 minutes.
To make the filling, melt the low-fat spread in a saucepan. Add the onion, carrot and celery, and sauté, stirring, for 2 minutes. Add the mushrooms, quorn, tomato purée, stock, herbs and a little salt and pepper, and stir well. Bring to the boil, reduce the heat and simmer gently for 20 minutes until the mixture is tender and the liquid is well reduced, stirring occasionally.
Spoon the mixture into the marrow rings, top each with a little cheese and bake uncovered in the oven for a further 25 minutes or until tender and golden. Serve hot.