  Using the six years of the spectroscopic monitoring of the luminous quasar HE
0435-4312 ($z=1.2231$) with the Southern African Large Telescope (SALT), in
combination with the photometric data (CATALINA, OGLE, SALTICAM, and BMT), we
determined the rest-frame time-delay of $296^{+13}_{-14}$ days between the MgII
broad-line emission and the ionizing continuum using seven different time-delay
inference methods. Artefact time-delay peaks and aliases were mitigated using
the bootstrap method, prior weighting probability function as well as by
analyzing unevenly sampled mock light curves. The MgII emission is considerably
variable with the fractional variability of $\sim 5.4\%$, which is comparable
to the continuum variability ($\sim 4.8\%$). Because of its high luminosity
($L_{3000}=10^{46.4}\,{\rm erg\,s^{-1}}$), the source is beneficial for a
further reduction of the scatter along the MgII-based radius-luminosity
relation and its extended versions, especially when the high-accreting
subsample that has an RMS scatter of $\sim 0.2$ dex is considered. This opens
up a possibility to use the high-accretor MgII-based radius-luminosity relation
for constraining cosmological parameters. With the current sample of 27
reverberation-mapped sources, the best-fit cosmological parameters
$(\Omega_{\rm m}, \Omega_{\Lambda})=(0.19; 0.62)$ are consistent with the
standard cosmological model within 1$\sigma$ confidence level.
