Anisotropic nanoparticles can be synthesized under kinetic control but may undergo subsequent shape changes due to atomic reorganization. Furthermore, their synthesis involves rapid steps, which are challenging to monitor in situ. Here, we show how a nanoemulsion of alkanethiols with an ethoxylated surfactant, easily prepared and metastable for months, can simultaneously prevent shape reorganization and arrest reaction kinetics. We illustrate this generic method on the silver nanoplates synthesized in concentrated acetic acid aqueous solutions, in which rapid shape reorganization occurs. We show that there exists an optimum thiol concentration corresponding to full coverage of all silver surface atoms, which can be simply calculated from particle dimensions. Furthermore, we demonstrate that arresting nanoparticle formation can be achieved within milliseconds using a tandem rapid mixers scheme in a continuous flow setup, allowing ex situ monitoring of the reaction.