  We present time-series spectroscopy and photometry of Gaia DR2
6097540197980557440, a new deeply-eclipsing hot subdwarf B (sdB) + M dwarf (dM)
binary. We discovered this object during the course of the Eclipsing Reflection
Effect Binaries from Optical Surveys (EREBOS) project, which aims to find new
eclipsing sdB+dM binaries (HW Vir systems) and increase the small sample of
studied systems. In addition to the primary eclipse, which is in excess of
$\sim$5 magnitudes in the optical, the light curve also shows features typical
for other HW Vir binaries such as a secondary eclipse and strong reflection
effect from the irradiated, cool companion. The orbital period is 0.127037 d
($\sim$3 hr), falling right at the peak of the orbital period distribution of
known HW Vir systems. Analysis of our time-series spectroscopy yields a radial
velocity semi-amplitude of $K_{\rm sdB}=100.0\pm2.0\,{\rm km\,s}^{-1}$, which
is amongst the fastest line-of-sight velocities found to date for an HW Vir
binary. State-of-the-art atmospheric models that account for deviations from
local thermodynamic equilibrium are used to determine the atmospheric
parameters of the sdB. Although we cannot claim a unique light curve modeling
solution, the best-fitting model has an sdB mass of $M_{\rm sdB} =
0.47\pm0.03\,M_{\odot}$ and a companion mass of $M_{\rm dM} =
0.18\pm0.01\,M_{\odot}$. The radius of the companion appears to be inflated
relative to theoretical mass-radius relationships, consistent with other known
HW Vir binaries. Additionally, the M dwarf is one of the most massive found to
date amongst this type of binary.
