  The tight correlations found between masses of supermassive black holes
(SMBHs) and the luminosities, total stellar masses, and velocity dispersions of
their host galaxies are often interpreted as a sign of their co-evolution.
Studying these correlations across redshift provides a powerful insight into
the evolutionary path followed by the quasar and its host galaxy. While the
mass of the black hole is accessible from single-epoch spectra, measuring the
mass of its host galaxy is challenging as the active nucleus largely overshines
its host. Here, we present a technique to probe quasar-host relations beyond
the local universe with strong gravitational lensing, hence overcoming the use
of stellar population models or velocity dispersion measurements, both prone to
degeneracies. We study in detail one of the three known cases of strong lensing
by a quasar to accurately measure the mass of its host and to infer a total
lensing mass of $\log_{10}(M_{\rm Tot, h}/M_{\odot}) = 10.27^{+0.06}_{-0.07}~$
within the Einstein radius of 1.2 kpc. The lensing measurement is more precise
than any other alternative technique and compatible with the local
$M_{BH}$-$M_{\star, h}$ scaling relation. The sample of such quasar-galaxy or
quasar-quasar lensing systems should reach a few hundreds with Euclid and
Rubin-LSST, thus enabling the application of such a method with statistically
significant sample sizes.
