Chop the onion small and mix with the bread-crumbs, sage, and milk or water. Peel the marrow and scoop out the pith and pips. Cut it in halves to do this, or, better still, if possible cut off one end and scoop out inside with a long knife. Tie the two halves together with clean string. Stuff the marrow and bake for 40 minutes on a well-greased tin. Lay some of the nutter on top and baste frequently until done. It should brown well. Serve with brown gravy or white sauce.