Marrow is the soft, fatty substance found inside the large bones of beef and veal. In the Victorian times, marrow was a popular delicacy, often scooped out with long narrow spoons and spread on toast. Nowadays, it is more often used as a base for flavouring stocks, soups and sauces.
Beef leg bones are readily available from butchers, already cut into pieces. They can be boiled together with other ingredients to make stocks or soups, or alternatively, they can be treated so that the marrow can be extracted. To do this, soak the bones overnight in cold water, then wrap them in foil and bake in a hot oven for 45 minutes. The soft marrow may then be simply scooped out.