  We present the results of the re-discovery of the decay $B^0 \to \pi^- \ell^+
\nu_\ell$ in 34.6 fb$^{-1}$ of Belle II data using hadronic $B$-tagging via the
Full Event Interpretation algorithm. We observe 21 signal events on a
background of 155 in a fit to the distribution of the square of the missing
mass, $M_{\mathrm{miss}}^2$, with a significance of 5.69$\sigma$, and determine
a total branching fraction of (1.58 $\pm$ 0.43$_{\mathrm{stat}}$ $\pm$
0.07$_{\mathrm{sys}}$) $\times 10^{-4}$.
