  In this work, we perform a full-spectrum fitting of 350 massive and passive
galaxies selected as cosmic chronometers from the LEGA-C ESO public survey to
derive their stellar ages, metallicities, and star-formation histories. We
extensively test our results by assessing their dependence on the possible
contribution of dust, calibration of noise and signal, and the use of
photometric data in addition to spectral information; we as well identify
indicators of the correct convergence of the results, including the shape of
the posterior distributions, the analysis of specific spectral features, and
the correct reproduction of the observed spectrum. We derive a clear
age-redshift trend compatible with the aging in a standard cosmological model,
showing a clear downsizing pattern, with more massive galaxies being formed at
higher redshift ($z_f\sim2.5$) with respect to lower massive ones ($z_f\sim2$).
From these data, we measure the differential aging of this population of cosmic
chronometers to derive a new measurement of the Hubble parameter, obtaining
$H(z=0.8) = 113.1 \pm 15.1 (\mathrm{stat.}) ^{+29.1}_{-11.3} (\mathrm{syst.})\
\mathrm{ km\ s^{-1}\ Mpc^{-1}}$. This analysis allows us for the first time to
compare the differential ages of cosmic chronometers measured on the same
sample with two completely different methods, the full-spectrum fit (this work)
and the analysis of Lick indices, known to correlate with the age and
metallicity of the stellar populations \citep{Borghi2022a}. Albeit an
understood offset in the absolute ages, the differential ages have proven to be
extremely compatible between the two methods, despite the very different data,
assumptions, and models considered, demonstrating the robustness of the method.
