  Optically quiescent quasars (OQQs) represent a recently systematised class of
infrared-luminous active galactic nuclei (AGN) which have galaxy-like optical
continua. They may represent an interesting, brief phase in the AGN life cycle,
e.g. either cocooned within high-covering-factor media or indicative of recent
triggering, though their nature remains unclear. Here, we present the first
targeted simultaneous X-ray observations of an OQQ, our previously identified
prototype, SDSS J075139.06+402811.2 at $z$=0.587. The source is significantly
detected over 0.5-16 keV with XMM-Newton and NuSTAR, unambiguously confirming
the presence of current accretion activity. Spectral modelling yields an
intrinsic luminosity $L_{\rm 2-10 keV}$ $\approx$4.4$ \times$10$^{43}$ erg
s$^{-1}$, well within the AGN regime, but underluminous relative to its
infrared power. It is lightly obscured, with log $N_{\rm H}$ [cm$^{-2}$]
$\approx$22.
