We report the spontaneous, in-situ synthesis of gold nanoparticles within onion-type multilamellar vesicles (MLV) using a simple and mild strategy. Monoolein, one of the MLV components, was used as reductant, without any additional chemical. Two different preparative pathways were employed that resulted in gold particle formation as asserted by UV-vis spectroscopy and transmission electron microscopy (TEM). When onions were prepared from a lamellar phase containing gold ions, nanoparticles with a rice grain shape and narrow size distribution (6 × 10 nm*nm) were formed, suggesting synthesis within the vesicles. When preformed onions were dispersed in a KAuCl4 solution, TEM and cryogenic temperature-transmission electron microscopy (cryo-TEM) analysis reveal that both extra- and intravesicular syntheses took place. Cryo-TEM imaging evidences the insertion of gold nanoparticles between MLV leaflets and the close coupling between particle morphology and the lamellar phase. A simple mechanism of particle growth within a lamellar phase is proposed that could explain the differences in nanoparticle size and shape observed between both preparative pathways.