There has been great interest in the incorporation of foreign materials into fullerene structures (C-60, nanotubes, nanoparticles, onions). This interest has been driven by the potential applications of the filled fullerenes, which lie in areas as diverse as optical, electronic, magnetic recording materials and nuclear medicine. In particular, the onion structures of extreme strength may offer excellent protection to their encapsulated nanomaterials for applications. Here, we describe controlled carbonisation of an iron-containing biomolecule, ferritin, at elevated temperatures. This simple technique produces macroscopic quantities of quasi-spherical fullerenic shells (onions) that encapsulate iron nanoparticles of a very narrow range of particle diameters.