In this study, a facile and controllable synthetic route for the fabrication of mushroom nanostructures (FexOy@PSD-SiO2) and their hollow derivatives has been established. The synthesis consists of partial coating of FexOy (Fe3O4 or Fe2O3) with polymer spheres, followed by attaching silica hemispheres. The surface-accessible FexOy nanoparticles on the Janus-type FexOy@PSD nanospheres are key for directing the growth of the silica hemisphere on the FexOy@PSD seeds. The size and the porosity of the silica hemispheres are tunable by adjusting the amount of TEOS used and addition of a proper surfactant in a Stober-type process. After the iron oxide cores were leached out with concentrated HCl, mushroom nanostructures with hollow interiors were obtained, where the morphology of the hollow interior faithfully replicates the shape of the iron oxide core previously filling this void. This synthetic strategy provides a controllable method for the large-scale preparation of asymmetric colloidal nanostructures which could serve as building blocks for the assembly of new types of nanostructures.