We report, for the first time, the organic functionalization of carbon onions. The functionalization was achieved via 1,3-dipolar cycloaddition of azomethine ylides to a mixture of amorphous carbon and concentric fullerenes. Once functionalized, the onions become soluble in organic solvents and it is possible to purify them from the rest of carbon. Their nonlinear optical properties have been studied in solution for application as broadband optical limiters. It was found that the functionalized onions are an example of soluble material that functions equally well in both the visible and the NIR regions.