  Measuring stellar velocity dispersions of quiescent galaxies beyond $z\sim2$
is observationally challenging. Such measurements require near-infrared spectra
with a continuum detection of at least moderate signal-to-noise, often
necessitating long integrations. In this paper, we present deep X-Shooter
spectroscopy of one of only two known gravitationally-lensed massive quiescent
galaxies at $z>2$. This galaxy is quadruply imaged, with the brightest images
magnified by a factor of $\sim5$. The total exposure time of our data is 9.8
hours on-source; however the magnification, and the slit placement encompassing
2 images, provides a total equivalent exposure time of 215 hours. From this
deep spectrum we measure a redshift ($z_{\mathrm{spec}}=2.756\pm0.001$), making
this one of the highest redshift quiescent galaxies that is spectroscopically
confirmed. We simultaneously fit both the spectroscopic and photometric data to
determine stellar population parameters and conclude this galaxy is relatively
young, intermediate-mass, consistent with low dust content, and has quenched
only relatively recently. This recent quenching is confirmed by strong Balmer
absorption, particularly $H\delta$. Remarkably, this proves that at least some
intermediate-mass galaxies have already quenched as early as $z\sim2.8$.
Additionally, we have measured a velocity dispersion
($\sigma=187\pm43~\mathrm{km/s}$), making this the highest-redshift quiescent
galaxy with a dispersion measurement. We confirm that this galaxy falls on the
same mass fundamental plane (MFP) as galaxies at z=2.2, consistent with little
to no evolution in the MFP up to z=2.8. Overall this galaxy is proof of
existence of intermediate-mass quenched galaxies in the distant universe, and
that lensing is a powerful tool for determining their properties with improved
accuracy.
