We report the occurrence of quasispherical Archimedean spiral carbon nanoparticles and their transformation into onion-like nested multishell fullerenes. Irradiation of electron beam onto nanosized carbon particles forms spiral nanoparticles prior to transformation into onions. In situ microscopic observation reveals that the spiral-onion conversion is reversible and progresses outward from the center of a particle. A scheme is presented whereby thermodynamically most stable onion configuration is reached through the reversible transformation with the spiral intermediate playing the role of shell-adjuster. The scheme encompasses the growth mechanism of single- as well as multishell fullerene formation.