Smoked ham hock
The cured and smoked lower leg joint of the pig, with bone, rind and plenty of connective tissue, which gives smoke and gelatine to the broth during long cooking.
Smoked ham hocks are sold by butchers and in many supermarkets, either at the meat counter or vacuum-packed in the chilled aisle, though smaller shops do not always stock them.
Substitute:
A piece of smoked pork belly or bone-in smoked pork loin brings the smoky, cured flavour, but the broth comes out thinner, because the rind and connective tissue that supply the gelatine are missing.