1. Halve the marrow lengthways and scoop out the seeds.
2. Lay each half side by side, in a well greased, large, shallow casserole.
3. Skin and chop the onion finely or grate it. Put into a basin with the beef, sausage-meat, if used, breadcrumbs, parsley, chives, Worcestershire sauce, and seasoning. Mix well.
4. Beat the egg until liquid and use it to bind the mixture.
5. Divide the stuffing between each marrow half. Cover the dish and bake in a moderate oven, 180°C, Gas 4, for 1 hour.
6. Strain off most of the liquid in the casserole.
7. Pour the cheese sauce over the marrow and bake for a further 20 minutes, uncovered, or until the sauce is golden-brown.