By introducing a conducting metal layer underneath a Fano resonant asymmetric ring/disk plasmonic nanocavity system, we demonstrate that electromagnetic fields can be strongly enhanced. These large electromagnetic fields extending deep into the medium are highly accessible and increase the interaction volume of analytes and optical fields. As a result, we demonstrate high refractive index sensitivities as large as 648 nm/RIU. By exciting Fano resonances with much sharper spectral features, as narrow as 9 nm, we experimentally show high figure of merits, 72, and reliable detection of protein mono- and bilayers. Furthermore, the conducting substrate enables strong interaction between fundamental and higher order modes of the system by minor structural asymmetry. This is very advantageous for experimental realization of systems supporting resonances with well defined Fano-like line shape without requiring challenging fabrication resolution. Exploiting conducting metallic substrates and the associated propagating surface plasmons at their interface could be extended to other Fano-resonant cavity geometries for improved biosensing performance.