Directed voltage-controlled assembly and disassembly of plasmonic nanoparticles (NPs) at electrified solid-electrolyte interfaces (SEI) offer novel opportunities for the creation of tuneable optical devices. We apply this concept to propose a fast electrotuneable, NP-based Fabry-Perot (FP) interferometer, comprising two parallel transparent electrodes in aqueous electrolyte, which form the polarizable SEI for directed assembly-disassembly of negatively charged NPs. An FP cavity between two reflective NP-monolayers assembled at such interfaces can be formed or deconstructed under positive or negative polarization of the electrodes, respectively. The inter-NP spacing may be tuned via applied potential. Since the intensity, wavelength, and linewidth of the reflectivity peak depend on the NP packing density, the transmission spectrum of the system can thus be varied. A detailed theoretical model of the system's optical response is presented, which shows excellent agreement with full-wave simulations. The tuning of the peak transmission wavelength and linewidth is investigated in detail. Design guidelines for such NP-based FP systems are established, where transmission characteristics can be electrotuned in-situ, without mechanically altering the cavity length.