  Symbiotic stars in which the symbiotic phenomenon is powered solely by
accretion, often at an average rate that is higher than in cataclysmic variable
stars, provide an important opportunity to diagnose boundary layers around
disk-accreting white dwarfs. Here we investigate SU Lyncis, a recently
discovered example of a purely accretion-powered symbiotic star, using the
first reliable X-ray spectroscopy, obtained with NuSTAR, and UV photometry
obtained with Swift. SU Lyn has hard, thermal, X-ray emission that is strongly
affected by a variable local absorber - that has little impact on the UV
emission. Its X-ray spectrum is described well using a plasma cooling from $k$T
$\approx$ 21 keV, with a 3 to 30 keV luminosity of approximately
4.9$\times$10$^{32}$ ergs s$^{-1}$. The spectrum is also consistent with the
presence of reflection with an amplitude of 1.0, although in that case, the
best-fit plasma temperature is 20-25% lower. The UV to X-ray luminosity ratio
of SU Lyn changed significantly between 2015 and 2016. We interpret this as a
consequence of a drop by almost 90% in the accretion rate. Whereas the UV
luminosity of the disk responded linearly, the luminosity of the optically thin
(hard X-ray) emission from the boundary layer remained roughly constant because
the boundary layer changed from partially optically thick to almost completely
optically thin. Under this interpretation, we place a lower limit on the white
dwarf mass of 0.7 M$_{\odot}$ (0.8 M$_{\odot}$ if we neglect reflection).
